- Joined
- Mar 1, 2000
- RO Number
- 6
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- 231
INTRODUCTION
Boat Power Tuning requires two very essential tools a good tachometer to read engine speed and a manifold vacuum gauge which will be an indicator of engine load. The vacuum gauge should read from O inches of Mercury to about 30 inches of Mercury. Propeller selection is varied, but both pitch and diameter changes can be made to improve overall speed and performance.
HOW ARE PROPELLERS SELECTED?
The boat manufacturer has a wide selection of propellers to choose from and does so to optimize Fewer (ski boats) or speed (cruisers). The first basic to remember is that propellers are listed by their diameter and pitch, in that order. So, a 16 X 18 propeller is 16 inches in diameter and has a pitch of 18 inches which means the distance the propeller would travel through a solid in one revolution. However, water is not a solid and propeller slip which is roughly 30% will effectively lower the pitch. Propellers are also right hand (clockwise rotation) or left hand (counter-clockwise rotation). More will be said about propellers in the body of this presentation; but for now, your propeller effectively transfers engine power to the water to move the boat.
I. SETTING BEST IDLE
Setting the best idle on your marine engine is a very essential step in power tuning your boat. Proper idle speed is very important for transmission/outdrive life, as well as dependable engine operation for docking maneuvers Check your manufacturer's specification for idle speed recommendations and do not exceed these speeds. Engines which are new have greater frictional losses. and, therefore, require higher "green" engine speeds. These engines will require resetting of the idle speed as the engine is broken in. Your boat will handle best with the slower of these two idle speeds.
Engine speed at idle depends on two factors: idle air adjustment (idle speed) and idle fuel adjustment, with a good engine speed tachometer, an engine vacuum gauge, and with the engine warmed up to proper operating temperature. Here is the procedure to follow:
A. First, set engine speed (RPM on tach) to manufacturer's specification.
Example: 600 RPM.
B. Then observe vacuum gauge with engine idling in neutral as idle fuel is set richer and /or leaner. To assure an initial idle fuel balance, with engine off, carefully turn each idle adjusting fuel screw in until it seats in the carburetor casting before setting the speed of Part A. Do not use a large screwdriver or excessive force since the seat may be damaged if the screw is turned in too hard. After establishing that each needle is seated, back out (counter clockwise) the screws one full turn each. (This will be the initial set point at which the speed is set with the idle speed screw.)
C. Balance each idle fuel screw until the highest engine vacuum is realized. It is highly likely that when this point is reached, the engine speed will be higher than that which was set in Step A, The speed screw should be reset until it reads true on the tach. and the idle fuel setting should again be repeated for best idle mixture at the new speed setting.
This "best idle speed" will give you control during docking maneuvers without adding undue stress to your transmission or outdrive. Depending on the propeller you have selected, you may find, especially on ski boats with high pitch props, that the boat will perform better while docking with an even slower than recommended idle speed. In any case, select a best idle speed for your hull and prop, and set the best fuel idle mixture to the procedure outlined above.
II. TUNING FOR BEST ACCELERATION
How your boat accelerates from idle (no wake speed) to cruise depends on a great variety of factors. some of which are:
-engine size
-boat weight and loading
-wetted hull condition (marine growth)
-water conditions
-fuel octane
-propeller condition
-ignition system
Assuming that the above list has been carefully addressed, the carburetor can be custom tuned to deliver best performance during accelerations by:
A. Accelerator Pump Calibration
The function of the accelerator pump is to deliver fuel to complement the idle and main system operation during transition operation. The start of fuel delivery from the pump is as critical as the amount of fuel delivery from the pump. With the engine off, have someone operate the helm throttle, while observing the pump discharge nozzles in the primary venturi. The slightest movement on the throttle linkage should result in a delivery of fuel from the discharge nozzle (pump "tip in"). If there is a lag, accelerations will be compromised. By adjusting the pump override screw, the pump can be set to deliver fuel as the throttle is opened. One caution here is to check the travel left at the pump lever at wide open throttle, where there should still be some pump diaphragm travel (approximately .020).
After adjusting the pump "tip in", take your boat out to an area where accelerations can be conducted in a safe manner, and accelerator pump capacity testing can be done. Pump Cam Kit Number 20-12 provides a wide range of accelerator pump cams which have cam ramp angles to vary fuel delivery versus throttle angle. Using the chart provided with the Kit, note which pump cam comes standard with your carburetor. With the help of a stop watch and a "third eye" on the tach, run the boat from idle to 2,000 RPM and check the elapsed time. Try cams with capacities above and below the standard cam and record the resulting elapsed times. The best idle to 2,000 RPM speed times will indicate the best cam for your vessel.
If, while running the above test, any bogs or hesitations occur, go no lower (or higher) on the pump cam selection. Your boat has very simply told you that it needs more (or less) fuel than last cam selected.
B. Correct Secondary Operation
Once you have tailored your accelerator pump to your vessel, the higher speed accelerations are most affected by secondary throttle opening. A secondary diaphragm "motor'' opens the secondary throttle according to a calibrated diaphragm spring The vacuum opens the valve, and the spring closes it.
A variety of these springs are available in Kit Number 20-13. The explanations contained in the spring kit rate the springs by "relative load". Simply put, the lighter the load, the earlier the secondary throttle opens. If your boat runs at 4,400 RPM at wide open throttle, a good rule of thumb would be to have your secondaries calibrated to be wide open at about 3,800 RPM engine speed. Of course, the secondary throttle will begin to open much sooner, about 2,900 RPM, and this opening will improve vessel acceleration and will be audible with the engine hatch open. If the secondaries open too soon (lower RPM), a bog or hesitation will result. If they open too late (higher RPM), fuel distribution to the engine suffers. It has been our development experience that springs heavier in load than the plain spring are not required in marine engines.
With stop watch in hand, and a safe course for acceleration, time your speeds from 2,000 RPM to wide open throttle for best secondary diaphragm spring selection.
Holley has developed a very handy Kit Number 20-59 which facilitates the removal of the cover and the replacement of the diaphragm spring, without removing the choke assembly. So, should conditions exist where continued change of that spring is indicated, try one of these kits for a real time saver.
C. Main Metering Jet Selection
Your Holley carburetor has been developed to supply fuel for correct engine operation, and should not require main metering jet changes for most acceleration calibrations. If, however, operating conditions or engine displacement to hull length combinations require fuel delivery changes, the changes should not result in main jet changes of more than four sizes up or down from the standard main let supplied.
II Power Valve Timing and Power Results
By definition. a power valve is a valve which opens at a given manifold vacuum and delivers fuel to match power requirements at or near maximum engine speeds. This valve is closed at high manifold vacuum and opens at a set lower vacuum. Fuel consumption when the valve is opened is from 25 to 40% higher than that at part throttle (valve closed). It is very important that at all cruise speeds, the power valve be closed for maximum fuel economy. Using the vacuum gauge from the idle set section, record the manifold vacuums and engine speeds at cruise speeds. A typical small block Chevrolet might cruise at 2,800 RPM at an 8" manifold vacuum. In this case. the power valve should be selected below on 8" vacuum range. Example: 5.5" Hg opening.
During accelerations, the power valve should open according to the load of the engine. A light throttle operation will be clean and economical without the power valve opening. The acceleration would be made from 17" hg to 10" hg manifold vacuums. Deeper throttle accelerations requiring more power from the engine will be made below 8" manifold vacuum and, consequently, require the power valve to be open. Holley's single stage power valves are available in l" hg vacuum increments from 2.5" hg to 10.5" hg ranges. Again, a good rule of thumb is to select a power valve number (opening vacuum) which is at least 2" hg lower than your vessel's cruising manifold vacuum.
III. TUNING FOR BEST CRUISE PERFORMANCE
(SECONDARY THROTTLES CLOSED)
A boat engine is much different than the engine in the family car. A vehicle rolling down the highway at 55 miles per hour needs perhaps 15 to 18 horsepower to maintain vehicle speed. A boat, on plane, cruising at 30 miles requires 10 times that horsepower. The marine engine works much harder than the family car engine and therefore, should be tuned for best performance at the speed it will most often experience.
A. Carburetor Sizing
The importance of correctly sizing the primary side of your marine carburetor cannot be over rated. The primary must be capable of passing enough fuel and air to satisfy engine load requirements during most cruise mode operations. If you use your boat in a manner different than that anticipated by the manufacturer, a different sized primary carburetor may be required. Racing and ski tournament boats are two examples of load application which may require larger sized primary and secondary carburetors. However. in most applications, the carburetor which is supplied by the engine/ boat builder will suffice for a variety of engine/vessel usage. It is well to bring your vessel to its best cruising speed, have an observer open the hatch and indicate the primary throttle position, for all cruising should be done on the primary only. Once this speed/ throttle position is noted. open the helm throttle until the secondaries just begin to open, noting this engine speed and its resulting vessel speed as maximum throttle for best part throttle fuel consumption.
B. Main Metering Jet Selection vs. Manifold Vacuum and RPM
Once a prudent cruise speed has been chosen for a particular vessel/engine combination, best main jet selection begins. Best manifold vacuum at cruise will be our indicator as it was in our idle set procedure The best main jet for part throttle (secondary throttle closed) will be that main jet which gives the highest manifold vacuum at a given engine speed (RPM) It follows that when best power is made, the throttle will be further closed. and the highest manifold vacuum will be recorded for a given vessel speed. When selecting best jet, care should be exercised not to go too lean. Engine temperature is a good indicator. If temperature begins to climb, use 20o F maximum, as a rule of thumb go no leaner in jet size. The richer end of the jet selection is harder to find, but best vacuum will be found in a 2 or 3 jet range, without serious implications to the engine.
C. Power Valve Selection
Simply put, the best cruise performance should be with the power valve closed. This will change depending on the load and speed of the vessel. Only in special cases, as in pulling skiers or other towing, will it be necessary for the primary power valve to be selected with a number above the cruise intake vacuum. In towing skiers, watch engine temperatures. If. after long distance pulling, the engine temperature begins to climb, the added fuel through the power valve will help cool the engine as well as deliver a bit more power. For example, a 351 Ford engine in an 18 foot ski boot would cruise pulling one skier at a 9" hg manifold vacuum. If this resulted in an increase of 20o F in engine operating temperature from the same boat without a skier. the power valve selected should be 9.5" hg. This will allow added fuel through the power valve and would lower the engine temperature to the accepted range.
D. Flame Arrestor Size Selection
There is an old adage in marine calibration and it goes like this: "All marine engines must have a Coast Guard approved flame arrestor, and some arrestors are not properly suited for performance marine engine use." Tests have shown that flame arrestors have more to do with top vessel speed than some propellers. The flame arrestor is supposed to limit flame propagation in the event of a backfire, but some flame arrestors also seriously limit the amount of air available to the engine. The result is lower vessel speed and lower engine operating speeds. Engineering tests on a 305 Chevrolet engine revealed an increase of 300 RPM at wide open throttle by changing to a larger, less restrictive flame arrestor. Suffice it to say. in the case of approved flame arrestors. "bigger is better.
E. Vessel Factors
Vessel factors affecting best cruise performance, in order of importance, are:
1. Vessel trim
The attitude of the vessel while underway is directly related to the amount of fuel consumed and horsepower required. By changing the trim of a boat by means of hydraulic trim tabs in the case of an inboard engine, or by tilting the outdrive in an I/O vessel and observing engine speed. one can select the best trim for most economical cruising.
2. Hull condition
Marine growth on underwater hulls is probably one of the most overlooked reasons for reasons for ass in cruise performance A growth of algae in fresh water or a case of barnacles in the ocean can slow a boat down by 5 mile per hour without ever showing itself. The wise boater keeps his hull clean and free from underwater growths and realizes his effort by increased performance and decreased fuel consumption.
3. Propeller sizing and condition
Propellers, as stated in the introduction. are the means by which horsepower is transmitted from the engine to the water. Volumes have been written on propeller selection, and it is not the intent of this guide to teach this subject. The load of the propeller increases as a cubic relation to speed. and is the limiting factor of final engine speed. Pitch and diameter can be varied to increase vessel speed, but normally. the propeller selected by the manufacturer covers the greatest range of uses by a particular hull design. Perhaps the condition of the propeller itself is more noteworthy to the boater. A small dent. a dulled edge caused by shallow water sand, or a knicked blade caused by floating debris can seriously affect propeller performance. These imperfections can be so small as to cause no vibration, the usual tip that underwater gear is faulty, yet can increase propeller slip by 10 to 15% A stainless steel propeller will, because it is much harder, last longer than an aluminum propeller and may be a boater's answer to damage. It will also because it is stronger, stay sharper. longer and keep propeller slip to a minimum.
IV. TUNING FOR WIDE OPEN THROTTLE
PERFORMANCE (SECONDARY THROTTLES OPEN)
This section deals with the helm throttle completely open and the Vessel on water calm and unobstructed to allow the boat to reach its wide open speed. Typically. this speed is somewhere between 4,000 and 5,200 RPM. Great care must be exercised during this testing to stay clear of traffic, and to watch for underwater hazards.
A. Carburetor Sizing
Your marine engine is basically a pump. which pumps air and fuel into the combustion chamber to be burned to produce power. Once the wide open throttle speed for a particular vessel is measured. a simple formula can help determine the required air flow of the carburetor.
Air flow in CFM = WOT RPM x Engine displacement in3 ft3
1728 x 2 REV in3
For a 454 CID marine engine which revs to 4,400 RPM:
Airflow = 4400x454 =578 CFM
1728 x 2
(assuming 100% volumertric efficiency)
Which simply says: This 454 engine used 578 CFM at a WOT cruise speed of 4,400 RPM assuming no pumping loses." For proper operation, a carburetor which delivers 600 CFM would be adequate for this engine: but, in practice, larger carburetors are used. It may be desirable to use a smaller carburetor or larger carburetor depending on whether part throttle or wide open throttle operation is most often used. Other factors which affect carburetor size selection are engine compartment temperature (causes air density changes) and exhaust system back pressure (causes intake charge dilution).
B. Secondary Diaphragm Operation
Your Holley marine carburetor has diaphragm operated secondary throttles. which means as indicated by engine speed and load, the air flow through the primary venturi actually opens the secondary at a predetermined point. As the secondary carburetor throttle begins to open, you will notice an increase in engine speed until the plates are wide open, and then the vessel will stabilize at its wide open throttle speed. The secondary opening point can be altered by changing the spring under the cover to either a lighter (earlier opening) or heavier (delayed opening) spring. The Spring Kit 20-13 will provide a spring range for almost any opening point. The spring is accessed by removing the secondary diaphragm housing, then removing top cover screws and the diaphragm spring is under that cover above the rubber diaphragm. As mentioned earlier, Holley makes a kit for a quick change of that spring without having to remove the diaphragm housing from the carburetor body (Kit No, 20-59). In the case of dual engine or multiple engine, to insure each engine goes to wide open secondary when the helm throttle is advanced, the secondary diaphragm housings must be all tuned to the same signal vacuum.
Manufacturing tolerances in the engine and carburetor manufacture cause sufficient differences that will be eliminated by connecting the vacuum diaphragms together. Holley Part Number 20-28 provides the means to connect each diaphragm with its counterpart on the other engine, to insure each assembly is responding to the same vacuum (balanced). Once connected with a vacuum hose, the engines will respond in unison to the helm throttle wide open position.
C. Power Valve Selection and Main Metering Jet Control
As discussed earlier in Section III. with the best main jet selected for cruise performance the power valve is now open to provide fuel for wide open power. The timing (opening point) of the power valve should be numerically greater than the manifold vacuum reading of the engine at wide open cruise. To check this, simply connect the vacuum gauge used in "Setting Best Idle" and read the wide open throttle vacuum.
Since the marine engine is really "putting out" at wide open, additional fuel calibration should not be attempted without consulting your dealer. Since the real horsepower output of your engine is near its rated maximum, fuel changes at that point could do more harm than good.
D. Flame Arrestor Sizing
Once you have recorded the manifold vacuum at wide open throttle, your flame arrestor can be rated. The importance of the flame suppression during a backfire is extremely critical for safe boat operation, but if your arrestor limits the air flow to your engine, it is, in effect, "throttling" your engine's power. As a rule of thumb, the following chart is offered to determine if your flame arrestor is too restrictive and a large arrestor should be considered.
Engine Maximum manifold vacuum
displacement without "throttling"
300 1.0"
350 11/2"
454 2"
502 2"
To select a "larger" flame arrestor, begin by determining the "effective area" of your existing arrestor. To do this, measure the outside diameter (d) of the element and the height (h) of the element between the base and cover. Then. apply this formula:
Effective Area = 3.14 x d x h IN2
To select a new arrestor, choose one which has an effective area at least 25% greater than the area of your standard arrestor. Remember, if clearance over the top of the arrestor is a problem, by choosing an arrestor with the same height but a larger diameter, you will increase the effective area without problem.
A special note here for those who intend to operate their vessels at or near wide open throttle for extended periods of time. You cannot have a flame arrestor too large. Crankcase fumes during wide open operation carry considerable oil in the fumes, and flame arrestor air flow reduction occurs as the elements are coated with oil. For the sake of maximizing power, use the larger arrestor for these high performance cases.
E. Propeller Limitations on Wide Open Throttle Performance
The propeller, as discussed earlier, transfers a load to the engine which effectively limits wide open throttle engine speed. Smaller diameter propellers turn faster than larger diameter propellers if both are of equal pitches. Changing to a propeller of a high pitch will slow the wide open throttle engine speed down. If you feel your engine needs to turn faster to increase your performance see your marine dealer for a propeller selection suggestion.
Earlier, it was mentioned that vibration is on indicator of propeller damage. Not all damage to propellers can be seen by the naked eye. Propeller blades can be sprung, and, therefore, require balancing by a responsible prop shop It is not wise to run an engine if propeller vibration is evident. Damage to outdrive bearings and shaft struts can occur rapidly and with disastrous results. Propellers should be reconditioned each year no matter if the encounter with underwater obstruction was severe or not.
F: Importance of Vessel Trim and Hull Condition
One trip to a boat yard in the spring reveals a good review of vessel trim and hull conditions. Good hydraulic trim tabs allow the boat operator to adjust the boat attitude for onboard loading as well as water conditions. It can be argued that proper loading stowage ca n aid in vessel trim, but wind and wave action can not be compensated without tabs. Wise owners of outdrive boats also install tabs as stern downward forces by the trim tabs are more effective in boat trim than changing the angle of thrust on the outdrive. Since tabs come in pairs. six degrees of adjustment are possible for all round boat trim.
Hull condition for maximum wide-open throttle performance means underwater surface condition and preparation. All marine growth must be eliminated to reduce drag. and scratches or hull nicks should be removed. The smother the hull is, the less drag is present, and the less horsepower is lost to the water.
HOW TO CARE FOR YOUR MARINE CARBURETOR FOR BEST PERFORMANCE
A. Corrosion Protection
Corrosion protection is more than just for looks. Exterior corrosion can affect the function of your carburetor. Holley marine carburetors come polished to a high luster to complement the dress items on today's marine engine. A few easy steps can keep your carburetor looking good and functioning properly. To keep your carburetor bright, protect the exterior surface finish by applying a clear acrylic spray which can be found in most art and craft stores.
Exterior springs on the throttle shafts and the idle speed and mixture screws can be protected by wiping them with a cloth which has been sprayed with a light oil such as WD. 40. Do not spray the oil on the springs as too much oil will attract dust and dirt which will later cause spring binding. In some cases, during salt water operation, airborne salt sprays find their way on to the carburetor. When this happens, only immediately rinsing with fresh water can stop the eventual corrosion. A point of interest is that the Holley marine carburetor rebuild kit will include replacement springs for these applications.
B. General Service
To insure proper service from a marine carburetor, the best advice is to keep it clean and free from corrosion. Keeping it clean means the outside should be inspected for residue from flame arrestor tube connections. Some manufacturers route the fumes from the valve covers to the element of the arrestor. Oil fumes condense and drip down on the carburetor where they attract dirt, or worse, cause caustic attack on the carburetor surfaces.
Other engine manufacturers vent the valve covers through a breather which vents the flow by products to carburetor area. The natural air flow to the flame arrestor keeps these vapors airborne where they finally settle on the carburetor. The exterior of the carburetor should be wiped clean to prevent residue build up. At the same time, the flame arrestor should be inspected and cleaned thoroughly with a good cleaner. Linkage on the carburetor should be cleaned to retain proper carburetor operation. The only area that lubrication should be used is on the throttle cable and the throttle ball to which it attaches. Use no other lubricants on carburetor moving parts.
Rubber parts such as hoses, gaskets, pump diaphragms and secondary diaphragms. should receive periodic inspection as these parts are attacked by ozone and deteriorate at high temperatures. Any hose which exhibits any surface cracking when bent 90o, should be replaced. Carburetor gaskets which weep fuel, as evidenced by stains on the casting surfaces, should be replaced. Pump diaphragms should be inspected regularly because during lay-up periods, the most detrimental materials in the fuel are in close contact with the accelerator pump diaphragm.
Holley has engineered very specific rebuilding kits for nearly all marine carburetors, and these kits are a good source of parts needed for regular maintenance.
C. Fuel Additives and Storage Fuel Stabilizers
The fuel tank of a marine vessel exists in an atmosphere of nearly 100% humidity for the life of the vessel. This means that during temperature excursions of a normal summer day, the fuel tank sees condensation of water in the fuel tank. This water being heavier (more dense) than the fuel settles to the fuel tank bottom where it accumulates. When the level of water reaches the bottom of the fuel pick-up tube, it proceeds through the system to the carburetor. Some fuel systems have water separators, but often these are not regularly serviced and water still finds its way to the carburetor fuel bowl. Water in the fuel bowl of any carburetor means real trouble. Corrosion of metering components often means expensive carburetor repair. Water in the combustion chamber means erratic idle, stalling, with loss of vessel control, and loss of power at cruise.
The best solution for these problems is to keep the fuel tank from having space in which moisture-laden air can collect and drop its water into the tank. If the fuel tank is kept full of fuel, this air can not collect, and therefore, most water is eliminated. Use of additives, which are usually methanol alcohol with a drop or two of isopropyl alcohol to "dry out" the fuel, only aggravates corrosion problems. Plus, these alcohol water mixtures also create problems with the rubber components and pump diaphragms. The answer to solving water problems is simple: keep your fuel tank(s) full, and replace your fuel /water separator regularly.
During winter lay up, most storage yards request that fuel tanks be filled to prevent excessive fuel vapors The fuel in these tanks should be treated with a fuel stabilizer to prevent the fuel from going "sour". "Sour gas" is the most damaging substance a fuel system can encounter. Fuel stabilizers should be used in concentrations according to the manufacturer's instructions.
D. Lay Up and Fitting Out
In preparation for winter lay up the carburetor should be cleaned externally and drained of all fuel by the following procedure. When lay up is imminent, and while the boat is still in the water, run the engine with transmission in neutral at about 1,000 RPM, Open access hatch and locate the fuel line shut off valve. Have ready a spray can of engine fogging oil. Remove flame arrestor while engine is at fast idle (1,000 RPM) and shut off fuel line valve. Position spray oil can over primary carburetor and wait for engine speed to change. An indication of running out of fuel will be an increase in idle speed. As idle speed increases, begin spraying oil into the primary carburetor. Engine speed will roughen, but it will not stall.
Continued spraying of the oil will allow all of the secondary carburetor fuel to be expended. When this occurs, idle will roughen more. Cease spraying the oil and the engine will stall. At this point. both fuel bowls are empty of all fuel except for a minimal amount of unusable fuel below the main jets. This fuel should cause no problems since it can not form varnish on any major metering restrictions. The excess oil which entered the combustion chambers will lubricate and protect the rings, piston, etc. from corrosion during lay up. Reinstall the flame arrestor, wipe off any spray which may be on the outside of the carburetor. and cover the carburetor and flame arrestor with an old towel or cloth. This will prevent those ever-inquisitive insects from building a winter home in your flame arrestor. Further engine lay up and winterizing should follow manufacturer's instructions.
In the spring, to fit out, remember first to open the fuel line valve which was shut off during lay up. When appropriate, namely in the water with engine cooling seacocks open and the protective towel removed, merely crank the engine until the bowls fill and engine fires. Then, let idle until warm. This also helps remove the protective oil from the combustion chambers as evidenced by smoke in the exhaust.
E. Extended Storage
If lay up extends longer than six months. additional carburetor attention is required. For these occasions, it is prudent to remove the carburetor and replace all gaskets, diaphragms, hoses, and inlet seats before recommissioning the engines. This will insure that your carburetor is reconditioned for continued trouble-free service. Holley rebuild marine carburetor kits are offered for these cases.
( Also see Extended Storage Start-up procedures)
F. Safety Precautions with Raw Fuel
At no time during your boating activities should you allow any liquid fuel to collect in or on your marine engine. Enclosed engine compartments will collect the vapors of spilled fuel and can experience mixtures, which are capable of dangerously explosive conditions. Do all carburetor rebuilding work where fuel might spill off the vessel. Exercise great care to identify and locate any fuel leakage. Use your nose each time you open the engine hatch. Don't depend on fuel vapor sensor systems. And always run your bilge blower at least five minutes before starting your engine.
Boat Power Tuning requires two very essential tools a good tachometer to read engine speed and a manifold vacuum gauge which will be an indicator of engine load. The vacuum gauge should read from O inches of Mercury to about 30 inches of Mercury. Propeller selection is varied, but both pitch and diameter changes can be made to improve overall speed and performance.
HOW ARE PROPELLERS SELECTED?
The boat manufacturer has a wide selection of propellers to choose from and does so to optimize Fewer (ski boats) or speed (cruisers). The first basic to remember is that propellers are listed by their diameter and pitch, in that order. So, a 16 X 18 propeller is 16 inches in diameter and has a pitch of 18 inches which means the distance the propeller would travel through a solid in one revolution. However, water is not a solid and propeller slip which is roughly 30% will effectively lower the pitch. Propellers are also right hand (clockwise rotation) or left hand (counter-clockwise rotation). More will be said about propellers in the body of this presentation; but for now, your propeller effectively transfers engine power to the water to move the boat.
I. SETTING BEST IDLE
Setting the best idle on your marine engine is a very essential step in power tuning your boat. Proper idle speed is very important for transmission/outdrive life, as well as dependable engine operation for docking maneuvers Check your manufacturer's specification for idle speed recommendations and do not exceed these speeds. Engines which are new have greater frictional losses. and, therefore, require higher "green" engine speeds. These engines will require resetting of the idle speed as the engine is broken in. Your boat will handle best with the slower of these two idle speeds.
Engine speed at idle depends on two factors: idle air adjustment (idle speed) and idle fuel adjustment, with a good engine speed tachometer, an engine vacuum gauge, and with the engine warmed up to proper operating temperature. Here is the procedure to follow:
A. First, set engine speed (RPM on tach) to manufacturer's specification.
Example: 600 RPM.
B. Then observe vacuum gauge with engine idling in neutral as idle fuel is set richer and /or leaner. To assure an initial idle fuel balance, with engine off, carefully turn each idle adjusting fuel screw in until it seats in the carburetor casting before setting the speed of Part A. Do not use a large screwdriver or excessive force since the seat may be damaged if the screw is turned in too hard. After establishing that each needle is seated, back out (counter clockwise) the screws one full turn each. (This will be the initial set point at which the speed is set with the idle speed screw.)
C. Balance each idle fuel screw until the highest engine vacuum is realized. It is highly likely that when this point is reached, the engine speed will be higher than that which was set in Step A, The speed screw should be reset until it reads true on the tach. and the idle fuel setting should again be repeated for best idle mixture at the new speed setting.
This "best idle speed" will give you control during docking maneuvers without adding undue stress to your transmission or outdrive. Depending on the propeller you have selected, you may find, especially on ski boats with high pitch props, that the boat will perform better while docking with an even slower than recommended idle speed. In any case, select a best idle speed for your hull and prop, and set the best fuel idle mixture to the procedure outlined above.
II. TUNING FOR BEST ACCELERATION
How your boat accelerates from idle (no wake speed) to cruise depends on a great variety of factors. some of which are:
-engine size
-boat weight and loading
-wetted hull condition (marine growth)
-water conditions
-fuel octane
-propeller condition
-ignition system
Assuming that the above list has been carefully addressed, the carburetor can be custom tuned to deliver best performance during accelerations by:
A. Accelerator Pump Calibration
The function of the accelerator pump is to deliver fuel to complement the idle and main system operation during transition operation. The start of fuel delivery from the pump is as critical as the amount of fuel delivery from the pump. With the engine off, have someone operate the helm throttle, while observing the pump discharge nozzles in the primary venturi. The slightest movement on the throttle linkage should result in a delivery of fuel from the discharge nozzle (pump "tip in"). If there is a lag, accelerations will be compromised. By adjusting the pump override screw, the pump can be set to deliver fuel as the throttle is opened. One caution here is to check the travel left at the pump lever at wide open throttle, where there should still be some pump diaphragm travel (approximately .020).
After adjusting the pump "tip in", take your boat out to an area where accelerations can be conducted in a safe manner, and accelerator pump capacity testing can be done. Pump Cam Kit Number 20-12 provides a wide range of accelerator pump cams which have cam ramp angles to vary fuel delivery versus throttle angle. Using the chart provided with the Kit, note which pump cam comes standard with your carburetor. With the help of a stop watch and a "third eye" on the tach, run the boat from idle to 2,000 RPM and check the elapsed time. Try cams with capacities above and below the standard cam and record the resulting elapsed times. The best idle to 2,000 RPM speed times will indicate the best cam for your vessel.
If, while running the above test, any bogs or hesitations occur, go no lower (or higher) on the pump cam selection. Your boat has very simply told you that it needs more (or less) fuel than last cam selected.
B. Correct Secondary Operation
Once you have tailored your accelerator pump to your vessel, the higher speed accelerations are most affected by secondary throttle opening. A secondary diaphragm "motor'' opens the secondary throttle according to a calibrated diaphragm spring The vacuum opens the valve, and the spring closes it.
A variety of these springs are available in Kit Number 20-13. The explanations contained in the spring kit rate the springs by "relative load". Simply put, the lighter the load, the earlier the secondary throttle opens. If your boat runs at 4,400 RPM at wide open throttle, a good rule of thumb would be to have your secondaries calibrated to be wide open at about 3,800 RPM engine speed. Of course, the secondary throttle will begin to open much sooner, about 2,900 RPM, and this opening will improve vessel acceleration and will be audible with the engine hatch open. If the secondaries open too soon (lower RPM), a bog or hesitation will result. If they open too late (higher RPM), fuel distribution to the engine suffers. It has been our development experience that springs heavier in load than the plain spring are not required in marine engines.
With stop watch in hand, and a safe course for acceleration, time your speeds from 2,000 RPM to wide open throttle for best secondary diaphragm spring selection.
Holley has developed a very handy Kit Number 20-59 which facilitates the removal of the cover and the replacement of the diaphragm spring, without removing the choke assembly. So, should conditions exist where continued change of that spring is indicated, try one of these kits for a real time saver.
C. Main Metering Jet Selection
Your Holley carburetor has been developed to supply fuel for correct engine operation, and should not require main metering jet changes for most acceleration calibrations. If, however, operating conditions or engine displacement to hull length combinations require fuel delivery changes, the changes should not result in main jet changes of more than four sizes up or down from the standard main let supplied.
II Power Valve Timing and Power Results
By definition. a power valve is a valve which opens at a given manifold vacuum and delivers fuel to match power requirements at or near maximum engine speeds. This valve is closed at high manifold vacuum and opens at a set lower vacuum. Fuel consumption when the valve is opened is from 25 to 40% higher than that at part throttle (valve closed). It is very important that at all cruise speeds, the power valve be closed for maximum fuel economy. Using the vacuum gauge from the idle set section, record the manifold vacuums and engine speeds at cruise speeds. A typical small block Chevrolet might cruise at 2,800 RPM at an 8" manifold vacuum. In this case. the power valve should be selected below on 8" vacuum range. Example: 5.5" Hg opening.
During accelerations, the power valve should open according to the load of the engine. A light throttle operation will be clean and economical without the power valve opening. The acceleration would be made from 17" hg to 10" hg manifold vacuums. Deeper throttle accelerations requiring more power from the engine will be made below 8" manifold vacuum and, consequently, require the power valve to be open. Holley's single stage power valves are available in l" hg vacuum increments from 2.5" hg to 10.5" hg ranges. Again, a good rule of thumb is to select a power valve number (opening vacuum) which is at least 2" hg lower than your vessel's cruising manifold vacuum.
III. TUNING FOR BEST CRUISE PERFORMANCE
(SECONDARY THROTTLES CLOSED)
A boat engine is much different than the engine in the family car. A vehicle rolling down the highway at 55 miles per hour needs perhaps 15 to 18 horsepower to maintain vehicle speed. A boat, on plane, cruising at 30 miles requires 10 times that horsepower. The marine engine works much harder than the family car engine and therefore, should be tuned for best performance at the speed it will most often experience.
A. Carburetor Sizing
The importance of correctly sizing the primary side of your marine carburetor cannot be over rated. The primary must be capable of passing enough fuel and air to satisfy engine load requirements during most cruise mode operations. If you use your boat in a manner different than that anticipated by the manufacturer, a different sized primary carburetor may be required. Racing and ski tournament boats are two examples of load application which may require larger sized primary and secondary carburetors. However. in most applications, the carburetor which is supplied by the engine/ boat builder will suffice for a variety of engine/vessel usage. It is well to bring your vessel to its best cruising speed, have an observer open the hatch and indicate the primary throttle position, for all cruising should be done on the primary only. Once this speed/ throttle position is noted. open the helm throttle until the secondaries just begin to open, noting this engine speed and its resulting vessel speed as maximum throttle for best part throttle fuel consumption.
B. Main Metering Jet Selection vs. Manifold Vacuum and RPM
Once a prudent cruise speed has been chosen for a particular vessel/engine combination, best main jet selection begins. Best manifold vacuum at cruise will be our indicator as it was in our idle set procedure The best main jet for part throttle (secondary throttle closed) will be that main jet which gives the highest manifold vacuum at a given engine speed (RPM) It follows that when best power is made, the throttle will be further closed. and the highest manifold vacuum will be recorded for a given vessel speed. When selecting best jet, care should be exercised not to go too lean. Engine temperature is a good indicator. If temperature begins to climb, use 20o F maximum, as a rule of thumb go no leaner in jet size. The richer end of the jet selection is harder to find, but best vacuum will be found in a 2 or 3 jet range, without serious implications to the engine.
C. Power Valve Selection
Simply put, the best cruise performance should be with the power valve closed. This will change depending on the load and speed of the vessel. Only in special cases, as in pulling skiers or other towing, will it be necessary for the primary power valve to be selected with a number above the cruise intake vacuum. In towing skiers, watch engine temperatures. If. after long distance pulling, the engine temperature begins to climb, the added fuel through the power valve will help cool the engine as well as deliver a bit more power. For example, a 351 Ford engine in an 18 foot ski boot would cruise pulling one skier at a 9" hg manifold vacuum. If this resulted in an increase of 20o F in engine operating temperature from the same boat without a skier. the power valve selected should be 9.5" hg. This will allow added fuel through the power valve and would lower the engine temperature to the accepted range.
D. Flame Arrestor Size Selection
There is an old adage in marine calibration and it goes like this: "All marine engines must have a Coast Guard approved flame arrestor, and some arrestors are not properly suited for performance marine engine use." Tests have shown that flame arrestors have more to do with top vessel speed than some propellers. The flame arrestor is supposed to limit flame propagation in the event of a backfire, but some flame arrestors also seriously limit the amount of air available to the engine. The result is lower vessel speed and lower engine operating speeds. Engineering tests on a 305 Chevrolet engine revealed an increase of 300 RPM at wide open throttle by changing to a larger, less restrictive flame arrestor. Suffice it to say. in the case of approved flame arrestors. "bigger is better.
E. Vessel Factors
Vessel factors affecting best cruise performance, in order of importance, are:
1. Vessel trim
The attitude of the vessel while underway is directly related to the amount of fuel consumed and horsepower required. By changing the trim of a boat by means of hydraulic trim tabs in the case of an inboard engine, or by tilting the outdrive in an I/O vessel and observing engine speed. one can select the best trim for most economical cruising.
2. Hull condition
Marine growth on underwater hulls is probably one of the most overlooked reasons for reasons for ass in cruise performance A growth of algae in fresh water or a case of barnacles in the ocean can slow a boat down by 5 mile per hour without ever showing itself. The wise boater keeps his hull clean and free from underwater growths and realizes his effort by increased performance and decreased fuel consumption.
3. Propeller sizing and condition
Propellers, as stated in the introduction. are the means by which horsepower is transmitted from the engine to the water. Volumes have been written on propeller selection, and it is not the intent of this guide to teach this subject. The load of the propeller increases as a cubic relation to speed. and is the limiting factor of final engine speed. Pitch and diameter can be varied to increase vessel speed, but normally. the propeller selected by the manufacturer covers the greatest range of uses by a particular hull design. Perhaps the condition of the propeller itself is more noteworthy to the boater. A small dent. a dulled edge caused by shallow water sand, or a knicked blade caused by floating debris can seriously affect propeller performance. These imperfections can be so small as to cause no vibration, the usual tip that underwater gear is faulty, yet can increase propeller slip by 10 to 15% A stainless steel propeller will, because it is much harder, last longer than an aluminum propeller and may be a boater's answer to damage. It will also because it is stronger, stay sharper. longer and keep propeller slip to a minimum.
IV. TUNING FOR WIDE OPEN THROTTLE
PERFORMANCE (SECONDARY THROTTLES OPEN)
This section deals with the helm throttle completely open and the Vessel on water calm and unobstructed to allow the boat to reach its wide open speed. Typically. this speed is somewhere between 4,000 and 5,200 RPM. Great care must be exercised during this testing to stay clear of traffic, and to watch for underwater hazards.
A. Carburetor Sizing
Your marine engine is basically a pump. which pumps air and fuel into the combustion chamber to be burned to produce power. Once the wide open throttle speed for a particular vessel is measured. a simple formula can help determine the required air flow of the carburetor.
Air flow in CFM = WOT RPM x Engine displacement in3 ft3
1728 x 2 REV in3
For a 454 CID marine engine which revs to 4,400 RPM:
Airflow = 4400x454 =578 CFM
1728 x 2
(assuming 100% volumertric efficiency)
Which simply says: This 454 engine used 578 CFM at a WOT cruise speed of 4,400 RPM assuming no pumping loses." For proper operation, a carburetor which delivers 600 CFM would be adequate for this engine: but, in practice, larger carburetors are used. It may be desirable to use a smaller carburetor or larger carburetor depending on whether part throttle or wide open throttle operation is most often used. Other factors which affect carburetor size selection are engine compartment temperature (causes air density changes) and exhaust system back pressure (causes intake charge dilution).
B. Secondary Diaphragm Operation
Your Holley marine carburetor has diaphragm operated secondary throttles. which means as indicated by engine speed and load, the air flow through the primary venturi actually opens the secondary at a predetermined point. As the secondary carburetor throttle begins to open, you will notice an increase in engine speed until the plates are wide open, and then the vessel will stabilize at its wide open throttle speed. The secondary opening point can be altered by changing the spring under the cover to either a lighter (earlier opening) or heavier (delayed opening) spring. The Spring Kit 20-13 will provide a spring range for almost any opening point. The spring is accessed by removing the secondary diaphragm housing, then removing top cover screws and the diaphragm spring is under that cover above the rubber diaphragm. As mentioned earlier, Holley makes a kit for a quick change of that spring without having to remove the diaphragm housing from the carburetor body (Kit No, 20-59). In the case of dual engine or multiple engine, to insure each engine goes to wide open secondary when the helm throttle is advanced, the secondary diaphragm housings must be all tuned to the same signal vacuum.
Manufacturing tolerances in the engine and carburetor manufacture cause sufficient differences that will be eliminated by connecting the vacuum diaphragms together. Holley Part Number 20-28 provides the means to connect each diaphragm with its counterpart on the other engine, to insure each assembly is responding to the same vacuum (balanced). Once connected with a vacuum hose, the engines will respond in unison to the helm throttle wide open position.
C. Power Valve Selection and Main Metering Jet Control
As discussed earlier in Section III. with the best main jet selected for cruise performance the power valve is now open to provide fuel for wide open power. The timing (opening point) of the power valve should be numerically greater than the manifold vacuum reading of the engine at wide open cruise. To check this, simply connect the vacuum gauge used in "Setting Best Idle" and read the wide open throttle vacuum.
Since the marine engine is really "putting out" at wide open, additional fuel calibration should not be attempted without consulting your dealer. Since the real horsepower output of your engine is near its rated maximum, fuel changes at that point could do more harm than good.
D. Flame Arrestor Sizing
Once you have recorded the manifold vacuum at wide open throttle, your flame arrestor can be rated. The importance of the flame suppression during a backfire is extremely critical for safe boat operation, but if your arrestor limits the air flow to your engine, it is, in effect, "throttling" your engine's power. As a rule of thumb, the following chart is offered to determine if your flame arrestor is too restrictive and a large arrestor should be considered.
Engine Maximum manifold vacuum
displacement without "throttling"
300 1.0"
350 11/2"
454 2"
502 2"
To select a "larger" flame arrestor, begin by determining the "effective area" of your existing arrestor. To do this, measure the outside diameter (d) of the element and the height (h) of the element between the base and cover. Then. apply this formula:
Effective Area = 3.14 x d x h IN2
To select a new arrestor, choose one which has an effective area at least 25% greater than the area of your standard arrestor. Remember, if clearance over the top of the arrestor is a problem, by choosing an arrestor with the same height but a larger diameter, you will increase the effective area without problem.
A special note here for those who intend to operate their vessels at or near wide open throttle for extended periods of time. You cannot have a flame arrestor too large. Crankcase fumes during wide open operation carry considerable oil in the fumes, and flame arrestor air flow reduction occurs as the elements are coated with oil. For the sake of maximizing power, use the larger arrestor for these high performance cases.
E. Propeller Limitations on Wide Open Throttle Performance
The propeller, as discussed earlier, transfers a load to the engine which effectively limits wide open throttle engine speed. Smaller diameter propellers turn faster than larger diameter propellers if both are of equal pitches. Changing to a propeller of a high pitch will slow the wide open throttle engine speed down. If you feel your engine needs to turn faster to increase your performance see your marine dealer for a propeller selection suggestion.
Earlier, it was mentioned that vibration is on indicator of propeller damage. Not all damage to propellers can be seen by the naked eye. Propeller blades can be sprung, and, therefore, require balancing by a responsible prop shop It is not wise to run an engine if propeller vibration is evident. Damage to outdrive bearings and shaft struts can occur rapidly and with disastrous results. Propellers should be reconditioned each year no matter if the encounter with underwater obstruction was severe or not.
F: Importance of Vessel Trim and Hull Condition
One trip to a boat yard in the spring reveals a good review of vessel trim and hull conditions. Good hydraulic trim tabs allow the boat operator to adjust the boat attitude for onboard loading as well as water conditions. It can be argued that proper loading stowage ca n aid in vessel trim, but wind and wave action can not be compensated without tabs. Wise owners of outdrive boats also install tabs as stern downward forces by the trim tabs are more effective in boat trim than changing the angle of thrust on the outdrive. Since tabs come in pairs. six degrees of adjustment are possible for all round boat trim.
Hull condition for maximum wide-open throttle performance means underwater surface condition and preparation. All marine growth must be eliminated to reduce drag. and scratches or hull nicks should be removed. The smother the hull is, the less drag is present, and the less horsepower is lost to the water.
HOW TO CARE FOR YOUR MARINE CARBURETOR FOR BEST PERFORMANCE
A. Corrosion Protection
Corrosion protection is more than just for looks. Exterior corrosion can affect the function of your carburetor. Holley marine carburetors come polished to a high luster to complement the dress items on today's marine engine. A few easy steps can keep your carburetor looking good and functioning properly. To keep your carburetor bright, protect the exterior surface finish by applying a clear acrylic spray which can be found in most art and craft stores.
Exterior springs on the throttle shafts and the idle speed and mixture screws can be protected by wiping them with a cloth which has been sprayed with a light oil such as WD. 40. Do not spray the oil on the springs as too much oil will attract dust and dirt which will later cause spring binding. In some cases, during salt water operation, airborne salt sprays find their way on to the carburetor. When this happens, only immediately rinsing with fresh water can stop the eventual corrosion. A point of interest is that the Holley marine carburetor rebuild kit will include replacement springs for these applications.
B. General Service
To insure proper service from a marine carburetor, the best advice is to keep it clean and free from corrosion. Keeping it clean means the outside should be inspected for residue from flame arrestor tube connections. Some manufacturers route the fumes from the valve covers to the element of the arrestor. Oil fumes condense and drip down on the carburetor where they attract dirt, or worse, cause caustic attack on the carburetor surfaces.
Other engine manufacturers vent the valve covers through a breather which vents the flow by products to carburetor area. The natural air flow to the flame arrestor keeps these vapors airborne where they finally settle on the carburetor. The exterior of the carburetor should be wiped clean to prevent residue build up. At the same time, the flame arrestor should be inspected and cleaned thoroughly with a good cleaner. Linkage on the carburetor should be cleaned to retain proper carburetor operation. The only area that lubrication should be used is on the throttle cable and the throttle ball to which it attaches. Use no other lubricants on carburetor moving parts.
Rubber parts such as hoses, gaskets, pump diaphragms and secondary diaphragms. should receive periodic inspection as these parts are attacked by ozone and deteriorate at high temperatures. Any hose which exhibits any surface cracking when bent 90o, should be replaced. Carburetor gaskets which weep fuel, as evidenced by stains on the casting surfaces, should be replaced. Pump diaphragms should be inspected regularly because during lay-up periods, the most detrimental materials in the fuel are in close contact with the accelerator pump diaphragm.
Holley has engineered very specific rebuilding kits for nearly all marine carburetors, and these kits are a good source of parts needed for regular maintenance.
C. Fuel Additives and Storage Fuel Stabilizers
The fuel tank of a marine vessel exists in an atmosphere of nearly 100% humidity for the life of the vessel. This means that during temperature excursions of a normal summer day, the fuel tank sees condensation of water in the fuel tank. This water being heavier (more dense) than the fuel settles to the fuel tank bottom where it accumulates. When the level of water reaches the bottom of the fuel pick-up tube, it proceeds through the system to the carburetor. Some fuel systems have water separators, but often these are not regularly serviced and water still finds its way to the carburetor fuel bowl. Water in the fuel bowl of any carburetor means real trouble. Corrosion of metering components often means expensive carburetor repair. Water in the combustion chamber means erratic idle, stalling, with loss of vessel control, and loss of power at cruise.
The best solution for these problems is to keep the fuel tank from having space in which moisture-laden air can collect and drop its water into the tank. If the fuel tank is kept full of fuel, this air can not collect, and therefore, most water is eliminated. Use of additives, which are usually methanol alcohol with a drop or two of isopropyl alcohol to "dry out" the fuel, only aggravates corrosion problems. Plus, these alcohol water mixtures also create problems with the rubber components and pump diaphragms. The answer to solving water problems is simple: keep your fuel tank(s) full, and replace your fuel /water separator regularly.
During winter lay up, most storage yards request that fuel tanks be filled to prevent excessive fuel vapors The fuel in these tanks should be treated with a fuel stabilizer to prevent the fuel from going "sour". "Sour gas" is the most damaging substance a fuel system can encounter. Fuel stabilizers should be used in concentrations according to the manufacturer's instructions.
D. Lay Up and Fitting Out
In preparation for winter lay up the carburetor should be cleaned externally and drained of all fuel by the following procedure. When lay up is imminent, and while the boat is still in the water, run the engine with transmission in neutral at about 1,000 RPM, Open access hatch and locate the fuel line shut off valve. Have ready a spray can of engine fogging oil. Remove flame arrestor while engine is at fast idle (1,000 RPM) and shut off fuel line valve. Position spray oil can over primary carburetor and wait for engine speed to change. An indication of running out of fuel will be an increase in idle speed. As idle speed increases, begin spraying oil into the primary carburetor. Engine speed will roughen, but it will not stall.
Continued spraying of the oil will allow all of the secondary carburetor fuel to be expended. When this occurs, idle will roughen more. Cease spraying the oil and the engine will stall. At this point. both fuel bowls are empty of all fuel except for a minimal amount of unusable fuel below the main jets. This fuel should cause no problems since it can not form varnish on any major metering restrictions. The excess oil which entered the combustion chambers will lubricate and protect the rings, piston, etc. from corrosion during lay up. Reinstall the flame arrestor, wipe off any spray which may be on the outside of the carburetor. and cover the carburetor and flame arrestor with an old towel or cloth. This will prevent those ever-inquisitive insects from building a winter home in your flame arrestor. Further engine lay up and winterizing should follow manufacturer's instructions.
In the spring, to fit out, remember first to open the fuel line valve which was shut off during lay up. When appropriate, namely in the water with engine cooling seacocks open and the protective towel removed, merely crank the engine until the bowls fill and engine fires. Then, let idle until warm. This also helps remove the protective oil from the combustion chambers as evidenced by smoke in the exhaust.
E. Extended Storage
If lay up extends longer than six months. additional carburetor attention is required. For these occasions, it is prudent to remove the carburetor and replace all gaskets, diaphragms, hoses, and inlet seats before recommissioning the engines. This will insure that your carburetor is reconditioned for continued trouble-free service. Holley rebuild marine carburetor kits are offered for these cases.
( Also see Extended Storage Start-up procedures)
F. Safety Precautions with Raw Fuel
At no time during your boating activities should you allow any liquid fuel to collect in or on your marine engine. Enclosed engine compartments will collect the vapors of spilled fuel and can experience mixtures, which are capable of dangerously explosive conditions. Do all carburetor rebuilding work where fuel might spill off the vessel. Exercise great care to identify and locate any fuel leakage. Use your nose each time you open the engine hatch. Don't depend on fuel vapor sensor systems. And always run your bilge blower at least five minutes before starting your engine.