- Joined
- Mar 1, 2000
- RO Number
- 6
- Messages
- 231
Troubleshooting With Vacuum Gauge
Steady reading between 15-21 inches at idle RPM = Normal
Extremely low reading, but steady at idle RPM = Vacuum leak; incorrect timing; under powered boat; faulty boat bottom
Fluctuates between high and low at idle RPM = Blown head gasket between two adjacent cylinders
Fluctuates 4 or 5 inches very slowly at idle RPM = Carburetor needs adjustment; spark plug gap too narrow; valves are sticking
Fluctuates rapidly at idle, steadies as RPM is increased = Valve guides are worn
Continuously fluctuates between low and normal reading at regular intervals at idle RPM = Burned or leaking valve
Readings drop to Zero as engine RPM is increased = Engine overload, clogged or restricted exhaust system
First, determine which engine system is causing the problem. To make an engine run, basic components fuel, spark (ignition), and compression are required. If all three components are present, the engine should run. If any one of the three are missing, weak, or arriving at the wrong time the engine will not run.
Determine if there is fuel present by looking down the carburetor venturi while actuating throttle. There should be a stream of fuel coming out of the accelerator pump nozzles if the carburetor has fuel. Check ignition system operation. Remove coil wire from tower on distributor cap. Hold coil wire near ground and check for spark while cranking engine over. Repeat procedure with spark plug wires. If there is spark at the spark plug wires, remove the spark plugs and make sure they are correct type and heat range, and not fouled or burned.
Run a compression check on engine, to make sure the engine is mechanically sound.
Not all problems are engine related, and probably many engine problems are exaggerated by other related items. The list below covers the often twisted paths that trouble shooting will take you. To start, look at the list below, and cross off the items that definitely "Are Not " the problem, and the list will seem much shorter.
Bow too low?
A. Improper drive unit trim angle
B. Improper weight distribution
C. Boat is under powered
D. Permanent or power hook in boat bottom
E. False bottom full of water
F. Improperly adjusted trim tabs (after planes)
G. Dirty boat bottom (marine growth)
Bow too high?
Improper drive unit trim angle
Propeller pitch too great
Dirty boat bottom (marine growth)
Poor running engine
Improper weight distribution
Rocker in boat bottom
False bottom full of water
Improperly adjusted trim tabs (after planes)
Propeller ventilating?
Drive unit installed too high on transom
Dirty or rough boat bottom
Damaged propeller; pitch too small; diameter too small
Keel located too close to propeller or too deep in the water
Water pickup or thru hull fittings located too ,close to propeller
Hook in boat bottom
Propeller plugged up with weeds
RPM Too High?
Propeller:
Damaged; pitch too low; diameter too small;
propeller hub slipping.
Boat:
Water pickup or thru hull fittings mounted too close to
propeller (ventilation);
keel located too close to propeller and/or too deep in
the water (ventilation).
Drive installed too high on transom; wrong gear ratio.
Operation:
Unit trimmed out too far.
Engine coupler slipping
RPM Too Low?
Propeller:
Damaged; pitch too great; diameter too great.
Boat:
Dirty or damaged bottom;
permanent or power hook in bottom;
false bottom full of water.
Drive installed too low on transom;
wrong gear ratio.
Operation:
Unit trimmed in too far.
Engine Cranks Over But Will Not Start Or Is Hard To Start
(No Spark)?
Moisture on ignition components
Distributor cap or spark plug wires arcing
Battery, electrical connections, damaged wiring
Faulty Ignition switch (check for 12 volts at coil)
Shift interrupter switch (Some sterndrive Models Only)
Shorted tachometer (disconnect and retry)
Ignition timing ( check if distributor is loose)
Spark plugs Fouled, burned, cracked porcelain
Spark plug wires faulty insulation, broken wires
Cracked or dirty distributor cap
Overheated rotor
Faulty ignition components
(Check components and wires, to and from)
Engine synchronizer (if equipped) hooked up series on purple ignition wire (dual engines only)
Synchronizers must be hooked up directly, coil terminal.
Low Voltage at coil when cranking (check battery and connections)
Engine Will Not Crank Over/Starter Inoperative?
Remote control lever not in neutral position
Battery charge low; damaged wiring; loose electrical connections
Circuit breaker tripped, Blown fuse (20 amp dash, 50 amp motor)
Ignition switch Slave solenoid
Faulty neutral start safety switch, open circuit
Starter solenoid
Starter motor
Engine Hydro-locked or seized
Mechanical engine malfunction (flywheel jammed, transmission seized)
Charging System Inoperative?
Loose or broken drive belt
Engine RPM too low on initial start, ( Rev engine to 1500 RPM)
Loose or corroded electrical connections
Faulty battery gauge ( Best way to test, try another gauge)
Battery will not accept charge ( see Battery Test Section)
Low electrolyte or failed battery
Poor ground to alternator (Don't trust the bracket)
Faulty alternator or regulator
Engine Runs Poorly At Idle?
Clogged flame arrestor
Improper idle fuel mixture adjustment
Engine floods at idle, (Bad needle and seat, float level off)
Problem in ignition system causing the engine to run rough
Idle mixture screw adjusted incorrectly
Incorrect float level drop
Moisture on ignition components (Cap or spark plug wires arcing)
Water in fuel
Low grade or stale fuel
Incorrect ignition timing
Automatic choke ( not opening )
Spark plugs Fouled, burned, cracked porcelain
Spark plug wires, Insulation breakdown, wires broken
Defective coil
Cracked or dirty distributor cap
Dirty carburetor
Vacuum leak at manifold or carburetor base
Fuel pump pressure Too low - can't supply the carburetor
Fuel pump pressure Too high - unseats the needle and seat
Low compression (Also check for blown head gasket)
Loose or worn distributor (Timing changes)
Water leaking into cylinders, Head gasket, exhaust manifold,
cracked head or valve seat
Loose or broken engine mounts
Engine Acceleration Is Poor?
Idle mixture screws Incorrect ignition timing
Incorrect distributor or amplifier advance curve
Accelerator pump (Check for stream of raw fuel from
accelerator pump discharge nozzle, when opening throttle
with engine shut off)
Cracked or dirty distributor cap or rotor
Vacuum leak Intake manifold or carburetor base
Spark plugs Fouled, burned; wrong heat range; cracked porcelain
Float adjustment
Dirty carburetor
Low compression
Engine Runs Poorly At High RPM?
Crankcase overfilled with oil, Check oil level with boat
at rest in the water.
Anti-siphon valve (if equipped) Restricting fuel supply
Plugged fuel tank vent
Fuel supply
Ignition timing
Low grade of fuel or water in the fuel
Spark plugs Fouled, burned, cracked porcelain, incorrect heat range
Spark plug wire, Poor insulation, broken wires
Distributor cap or rotor Dirty or cracked Coil
Distributor Excessive play in shaft
Engine overheating Refer to "Engine Overheats"
Low compression (Worn valves, rings, cylinders, etc.)
Restricted exhaust
Dirty Flame arrestor ( Wrong One?)
Not enough air flow in engine room ( open Hatch)
Sterndrive malfunction or seizing
Fuel System Rich?
Warm engine carburetor percolation, ( Fuel boils out of float
bowl when shut off and warm. Floods intake manifold.)
Clogged flame arrestor
Automatic choke not opening
Float adjustment
Float leaks or is saturated with fuel
Needle and seat leaking
Carburetor gaskets leaking
Excessive fuel pump pressure (Unseats needle and seat)
Cracked or porous carburetor body
Fuel System Lean?
Empty fuel tank
Fuel shut-off valve closed (if equipped)
Vapor lock Engine will not start after warm engine shut down
Automatic choke Stuck open, wrong adjustment
Fuel tank vent plugged Engine will start initially. After a
short time of running, engine will stall and will not
restart for a period of time. Can verify it is a vent
problem by running engine with filler cap loose.
Fill cap will act as a vent.
Air leak on suction side of fuel system (Sucks air into fuel
system reducing fuel volume)
Plugged or pinched fuel line
Fuel pump Low pump pressure
Carburetor Malfunction?
Flooding:
Needle and seat
Float adjustment,
Saturated float,
Gaskets leaking,
Cracked fuel bowl,
Fuel percolation,
Automatic choke,
Rough idle:
Idle RPM too low
Idle mixture screws
Idle passages dirty
Throttle valves not closing
Engine flooding
Vacuum leak
Throttle body heat passages plugged
Hesitation or acceleration flatness:
Accelerator pump
Leaking gaskets
Automatic choke
Power piston or power valve
Throttle valves
Throttle body heat passages plugged
Main metering jets
Float adjustment
Secondary air valve wind-up
Engine surges:
Main metering jets
Leaking gaskets
Float adjustment
Saturated float
Power piston or valve
Throttle valves
Low top speed or lack of power:
Power piston or valve
Float adjustment
Main metering jets
Leaking gaskets
Poor cold engine operation:
Idle RPM too low
Idle mixture screws
Throttle valves
Automatic choke
Engine flooding
Engine stalls:
Idle RPM too low
Idle mixture screws
Engine flooding
Automatic choke
Dirt in carburetor
Accelerator pump
Leaking gaskets
Poor Fuel Economy?
Fuel leaks
Operator habits
Prolonged idling;
slow acceleration;
failure to cut back on throttle once boat is on plane;
boat over loaded;
uneven weight distribution
Engine laboring?
Bent, damaged, or wrong propeller.
Water test boat for proper operating
RPM at wide open throttle
Clogged flame arrestor
Engine compartment sealed too tight
Not enough air for engine to run properly
Boat bottom Dirty (marine growth), hook, rocker
Carburetor Idle mixture settings, accelerator pump adjustments,
linkage binding, choke adjustment, carburetor flooding over,
main fuel jets
Improper fuel
Crankcase ventilation system not working
Engine needs tune-up
Engine running too cold or too hot
Plugged or restricted exhaust
Engine Low compression
Steady reading between 15-21 inches at idle RPM = Normal
Extremely low reading, but steady at idle RPM = Vacuum leak; incorrect timing; under powered boat; faulty boat bottom
Fluctuates between high and low at idle RPM = Blown head gasket between two adjacent cylinders
Fluctuates 4 or 5 inches very slowly at idle RPM = Carburetor needs adjustment; spark plug gap too narrow; valves are sticking
Fluctuates rapidly at idle, steadies as RPM is increased = Valve guides are worn
Continuously fluctuates between low and normal reading at regular intervals at idle RPM = Burned or leaking valve
Readings drop to Zero as engine RPM is increased = Engine overload, clogged or restricted exhaust system
First, determine which engine system is causing the problem. To make an engine run, basic components fuel, spark (ignition), and compression are required. If all three components are present, the engine should run. If any one of the three are missing, weak, or arriving at the wrong time the engine will not run.
Determine if there is fuel present by looking down the carburetor venturi while actuating throttle. There should be a stream of fuel coming out of the accelerator pump nozzles if the carburetor has fuel. Check ignition system operation. Remove coil wire from tower on distributor cap. Hold coil wire near ground and check for spark while cranking engine over. Repeat procedure with spark plug wires. If there is spark at the spark plug wires, remove the spark plugs and make sure they are correct type and heat range, and not fouled or burned.
Run a compression check on engine, to make sure the engine is mechanically sound.
Not all problems are engine related, and probably many engine problems are exaggerated by other related items. The list below covers the often twisted paths that trouble shooting will take you. To start, look at the list below, and cross off the items that definitely "Are Not " the problem, and the list will seem much shorter.
Bow too low?
A. Improper drive unit trim angle
B. Improper weight distribution
C. Boat is under powered
D. Permanent or power hook in boat bottom
E. False bottom full of water
F. Improperly adjusted trim tabs (after planes)
G. Dirty boat bottom (marine growth)
Bow too high?
Improper drive unit trim angle
Propeller pitch too great
Dirty boat bottom (marine growth)
Poor running engine
Improper weight distribution
Rocker in boat bottom
False bottom full of water
Improperly adjusted trim tabs (after planes)
Propeller ventilating?
Drive unit installed too high on transom
Dirty or rough boat bottom
Damaged propeller; pitch too small; diameter too small
Keel located too close to propeller or too deep in the water
Water pickup or thru hull fittings located too ,close to propeller
Hook in boat bottom
Propeller plugged up with weeds
RPM Too High?
Propeller:
Damaged; pitch too low; diameter too small;
propeller hub slipping.
Boat:
Water pickup or thru hull fittings mounted too close to
propeller (ventilation);
keel located too close to propeller and/or too deep in
the water (ventilation).
Drive installed too high on transom; wrong gear ratio.
Operation:
Unit trimmed out too far.
Engine coupler slipping
RPM Too Low?
Propeller:
Damaged; pitch too great; diameter too great.
Boat:
Dirty or damaged bottom;
permanent or power hook in bottom;
false bottom full of water.
Drive installed too low on transom;
wrong gear ratio.
Operation:
Unit trimmed in too far.
Engine Cranks Over But Will Not Start Or Is Hard To Start
(No Spark)?
Moisture on ignition components
Distributor cap or spark plug wires arcing
Battery, electrical connections, damaged wiring
Faulty Ignition switch (check for 12 volts at coil)
Shift interrupter switch (Some sterndrive Models Only)
Shorted tachometer (disconnect and retry)
Ignition timing ( check if distributor is loose)
Spark plugs Fouled, burned, cracked porcelain
Spark plug wires faulty insulation, broken wires
Cracked or dirty distributor cap
Overheated rotor
Faulty ignition components
(Check components and wires, to and from)
Engine synchronizer (if equipped) hooked up series on purple ignition wire (dual engines only)
Synchronizers must be hooked up directly, coil terminal.
Low Voltage at coil when cranking (check battery and connections)
Engine Will Not Crank Over/Starter Inoperative?
Remote control lever not in neutral position
Battery charge low; damaged wiring; loose electrical connections
Circuit breaker tripped, Blown fuse (20 amp dash, 50 amp motor)
Ignition switch Slave solenoid
Faulty neutral start safety switch, open circuit
Starter solenoid
Starter motor
Engine Hydro-locked or seized
Mechanical engine malfunction (flywheel jammed, transmission seized)
Charging System Inoperative?
Loose or broken drive belt
Engine RPM too low on initial start, ( Rev engine to 1500 RPM)
Loose or corroded electrical connections
Faulty battery gauge ( Best way to test, try another gauge)
Battery will not accept charge ( see Battery Test Section)
Low electrolyte or failed battery
Poor ground to alternator (Don't trust the bracket)
Faulty alternator or regulator
Engine Runs Poorly At Idle?
Clogged flame arrestor
Improper idle fuel mixture adjustment
Engine floods at idle, (Bad needle and seat, float level off)
Problem in ignition system causing the engine to run rough
Idle mixture screw adjusted incorrectly
Incorrect float level drop
Moisture on ignition components (Cap or spark plug wires arcing)
Water in fuel
Low grade or stale fuel
Incorrect ignition timing
Automatic choke ( not opening )
Spark plugs Fouled, burned, cracked porcelain
Spark plug wires, Insulation breakdown, wires broken
Defective coil
Cracked or dirty distributor cap
Dirty carburetor
Vacuum leak at manifold or carburetor base
Fuel pump pressure Too low - can't supply the carburetor
Fuel pump pressure Too high - unseats the needle and seat
Low compression (Also check for blown head gasket)
Loose or worn distributor (Timing changes)
Water leaking into cylinders, Head gasket, exhaust manifold,
cracked head or valve seat
Loose or broken engine mounts
Engine Acceleration Is Poor?
Idle mixture screws Incorrect ignition timing
Incorrect distributor or amplifier advance curve
Accelerator pump (Check for stream of raw fuel from
accelerator pump discharge nozzle, when opening throttle
with engine shut off)
Cracked or dirty distributor cap or rotor
Vacuum leak Intake manifold or carburetor base
Spark plugs Fouled, burned; wrong heat range; cracked porcelain
Float adjustment
Dirty carburetor
Low compression
Engine Runs Poorly At High RPM?
Crankcase overfilled with oil, Check oil level with boat
at rest in the water.
Anti-siphon valve (if equipped) Restricting fuel supply
Plugged fuel tank vent
Fuel supply
Ignition timing
Low grade of fuel or water in the fuel
Spark plugs Fouled, burned, cracked porcelain, incorrect heat range
Spark plug wire, Poor insulation, broken wires
Distributor cap or rotor Dirty or cracked Coil
Distributor Excessive play in shaft
Engine overheating Refer to "Engine Overheats"
Low compression (Worn valves, rings, cylinders, etc.)
Restricted exhaust
Dirty Flame arrestor ( Wrong One?)
Not enough air flow in engine room ( open Hatch)
Sterndrive malfunction or seizing
Fuel System Rich?
Warm engine carburetor percolation, ( Fuel boils out of float
bowl when shut off and warm. Floods intake manifold.)
Clogged flame arrestor
Automatic choke not opening
Float adjustment
Float leaks or is saturated with fuel
Needle and seat leaking
Carburetor gaskets leaking
Excessive fuel pump pressure (Unseats needle and seat)
Cracked or porous carburetor body
Fuel System Lean?
Empty fuel tank
Fuel shut-off valve closed (if equipped)
Vapor lock Engine will not start after warm engine shut down
Automatic choke Stuck open, wrong adjustment
Fuel tank vent plugged Engine will start initially. After a
short time of running, engine will stall and will not
restart for a period of time. Can verify it is a vent
problem by running engine with filler cap loose.
Fill cap will act as a vent.
Air leak on suction side of fuel system (Sucks air into fuel
system reducing fuel volume)
Plugged or pinched fuel line
Fuel pump Low pump pressure
Carburetor Malfunction?
Flooding:
Needle and seat
Float adjustment,
Saturated float,
Gaskets leaking,
Cracked fuel bowl,
Fuel percolation,
Automatic choke,
Rough idle:
Idle RPM too low
Idle mixture screws
Idle passages dirty
Throttle valves not closing
Engine flooding
Vacuum leak
Throttle body heat passages plugged
Hesitation or acceleration flatness:
Accelerator pump
Leaking gaskets
Automatic choke
Power piston or power valve
Throttle valves
Throttle body heat passages plugged
Main metering jets
Float adjustment
Secondary air valve wind-up
Engine surges:
Main metering jets
Leaking gaskets
Float adjustment
Saturated float
Power piston or valve
Throttle valves
Low top speed or lack of power:
Power piston or valve
Float adjustment
Main metering jets
Leaking gaskets
Poor cold engine operation:
Idle RPM too low
Idle mixture screws
Throttle valves
Automatic choke
Engine flooding
Engine stalls:
Idle RPM too low
Idle mixture screws
Engine flooding
Automatic choke
Dirt in carburetor
Accelerator pump
Leaking gaskets
Poor Fuel Economy?
Fuel leaks
Operator habits
Prolonged idling;
slow acceleration;
failure to cut back on throttle once boat is on plane;
boat over loaded;
uneven weight distribution
Engine laboring?
Bent, damaged, or wrong propeller.
Water test boat for proper operating
RPM at wide open throttle
Clogged flame arrestor
Engine compartment sealed too tight
Not enough air for engine to run properly
Boat bottom Dirty (marine growth), hook, rocker
Carburetor Idle mixture settings, accelerator pump adjustments,
linkage binding, choke adjustment, carburetor flooding over,
main fuel jets
Improper fuel
Crankcase ventilation system not working
Engine needs tune-up
Engine running too cold or too hot
Plugged or restricted exhaust
Engine Low compression