Hey everyone;
Long time no post! I apologize in advance for this very lengthy post, but there's a lot of information I'd like to convey... (If you're still reading this, now would be a good time to grab a beverage!)
So it's been almost a year since I rebuilt the Mercruiser 7.4L in my son's 1995 Regal Commodore 265. Unfortunately, we are still having the same problems. (Initial rebuild thread here: http://www.boatered.com/forum/topic.asp?TOPIC_ID=179442 )
Engine starts and idles perfectly. Throttles up to around 3000 RPM perfectly. At that point the secondaries begin to open on the carb, and the engine just won't rev up any more. We have replaced all fuel lines and fittings all the way back to the tank. No help. Then we installed a new electric fuel pump. No help. New fuel/water separator. Nothing. And finally, a new carburetor (Webber 750 CFM 4bbl). Still nothing.
Here is a video looking down the throat of the carb while we throttle up.
https://www.youtube.com/watch?v=HJWAOHRXpcs
You can see the fuel pattern on the primaries (near the bottom of the video), and that looks fine. But as the secondaries open, you'll see that there isn't much fuel coming out. You can also hear the note of the engine change (like it's starting to bog). Note that this video was shot with the old carb in place, but with the new one the results are exactly the same, which tells me it's not a carb or fuel delivery problem. (Please correct me if I've missed something.)
Before I continue, I should point out that every once in a great while the engine *WOULD* rev up past 3200. We've actually gotten it up to 4200 RPM a few times during the ~ 25 hrs we've put on the boat trying to fix this.
Here is a video shot down the throat of the carb during one of those rare "successful" run-ups: https://youtu.be/O4rZ3iHxh6Q You can see that the secondary fuel spray pattern isn't great, but it's much better than what you see in the first video above. (Secondaries are on the left in this video.)
But invariably once you throttle back down, you are back to the same problem when you try to throttle up again. You just can't get past 3000 to 3200. Also, as you see in the video, even when you do get it to rev up past 4000 RPM, it's clear that the fuel pattern through the secondaries still isn't perfect. It's like there isn't quite enough vacuum to pull the fuel into the carb...
So anyway, my next step was to check the intake manifold vacuum. (RO Sandy suggested this in my previous thread.) I hooked up my gauge to the 3/8 inch hole on the STBD side of the intake manifold, just aft of the carb. I've got a video showing vacuum from idle up through a WOT run-up in the water. Have a look:
https://www.youtube.com/watch?v=2KSrqCpgRJU
Apologies in advance for the "F" bomb at the end of the video... I did not expect that we would be able to get the RPMs up past 3200 on that run. Guess we got lucky! However, when we repeated the test just a minute later, we once again were back to a hard limit of around 3200.
The thing that concerns me about the vacuum reading is that at idle we only had 13 inches... If I understand things correctly, we should have at least 15 inches, if not more. Also, there is a little fluctuation in the needle as we are throttling up, even though it smooths out later on. Not sure if that is significant or not.
Does anyone think the low manifold vacuum at idle is a problem? And if so, what should I be looking at next?
Finally, just to recap the history of this engine: We suspect that we had an ignition system failure (either cross-firing or bad module) that caused extreme pre-ignition / detonation on the # 5 piston while running. This took a chunk out of the piston and also blew the head gasket on that side. (pic of the damage: http://imgur.com/AtoBrAA )
Pulled the engine last summer and installed new pistons, rings, rod bearings, oil pump, exhaust manifolds and elbows. Left the cam, crank, and timing set alone. Did a valve job on the heads (including new seals). Replaced ignition coil, ignition sensor in distributor, rotor and cap, plugs and plug wires. New fuel pump, fuel lines, water separator, and carburetor later on once RPM problem became apparent.
We have at least 25 hrs on the engine since the rebuild at this point, and we are just about out of ideas. I've run a bore-scope camera through the outdrive exhaust passages and up through the Y-pipe to ensure there are no blockages. We even removed the exhaust flappers in the Y pipe just to be sure it wasn't a back-pressure issue. No help.
The rest of the boat is essentially un-changed. Same outdrive (Bravo 3 with 26" props), same boat load-out, no hull fouling, no water-logging, weight unchanged. (It lives on a trailer.) And like I mentioned above, every once in a great while it *will* rev up to around 4000 or so, usually on the very first run-up of the day. But never more than once, and even then only infrequently.
And it really should get to 4600 (like it did before the rebuild).
I'm convinced that the inadequate fuel flow through the secondaries is a problem, but I believe this is a SYMPTOM of a bigger problem. I don't think the carb or the fuel system is the culprit, because it did the same thing with all the old fuel system parts, and now with everything new it's doing the exact same thing. Which is why I'm starting to focus on the manifold vacuum, but that just raises more questions...
Sure hope someone here has some advice, because I just don't know what else to try...
Adam
EDIT: Forgot to add that base ignition timing is set at 8 degrees BTDC and the electronic timing advance is functioning. (Verified with a timing light while operating under load). Getting up to 20 degrees of advance beyond base, which appears to be correct.
EDIT2: For posterity, I'm adding this addendum: The ultimate solution was to replace the ignition control module. It was failing to properly advance the timing when the engine was throttled up under load. This was hard to see, as I never kept the timing light on for more than a few seconds during early troubleshooting and the module would only fail intermittently. For the complete history, read the thread through to the end where the solution was eventually discovered (after a lot of work).
Long time no post! I apologize in advance for this very lengthy post, but there's a lot of information I'd like to convey... (If you're still reading this, now would be a good time to grab a beverage!)
So it's been almost a year since I rebuilt the Mercruiser 7.4L in my son's 1995 Regal Commodore 265. Unfortunately, we are still having the same problems. (Initial rebuild thread here: http://www.boatered.com/forum/topic.asp?TOPIC_ID=179442 )
Engine starts and idles perfectly. Throttles up to around 3000 RPM perfectly. At that point the secondaries begin to open on the carb, and the engine just won't rev up any more. We have replaced all fuel lines and fittings all the way back to the tank. No help. Then we installed a new electric fuel pump. No help. New fuel/water separator. Nothing. And finally, a new carburetor (Webber 750 CFM 4bbl). Still nothing.
Here is a video looking down the throat of the carb while we throttle up.
https://www.youtube.com/watch?v=HJWAOHRXpcs
You can see the fuel pattern on the primaries (near the bottom of the video), and that looks fine. But as the secondaries open, you'll see that there isn't much fuel coming out. You can also hear the note of the engine change (like it's starting to bog). Note that this video was shot with the old carb in place, but with the new one the results are exactly the same, which tells me it's not a carb or fuel delivery problem. (Please correct me if I've missed something.)
Before I continue, I should point out that every once in a great while the engine *WOULD* rev up past 3200. We've actually gotten it up to 4200 RPM a few times during the ~ 25 hrs we've put on the boat trying to fix this.
Here is a video shot down the throat of the carb during one of those rare "successful" run-ups: https://youtu.be/O4rZ3iHxh6Q You can see that the secondary fuel spray pattern isn't great, but it's much better than what you see in the first video above. (Secondaries are on the left in this video.)
But invariably once you throttle back down, you are back to the same problem when you try to throttle up again. You just can't get past 3000 to 3200. Also, as you see in the video, even when you do get it to rev up past 4000 RPM, it's clear that the fuel pattern through the secondaries still isn't perfect. It's like there isn't quite enough vacuum to pull the fuel into the carb...
So anyway, my next step was to check the intake manifold vacuum. (RO Sandy suggested this in my previous thread.) I hooked up my gauge to the 3/8 inch hole on the STBD side of the intake manifold, just aft of the carb. I've got a video showing vacuum from idle up through a WOT run-up in the water. Have a look:
https://www.youtube.com/watch?v=2KSrqCpgRJU
Apologies in advance for the "F" bomb at the end of the video... I did not expect that we would be able to get the RPMs up past 3200 on that run. Guess we got lucky! However, when we repeated the test just a minute later, we once again were back to a hard limit of around 3200.
The thing that concerns me about the vacuum reading is that at idle we only had 13 inches... If I understand things correctly, we should have at least 15 inches, if not more. Also, there is a little fluctuation in the needle as we are throttling up, even though it smooths out later on. Not sure if that is significant or not.
Does anyone think the low manifold vacuum at idle is a problem? And if so, what should I be looking at next?
Finally, just to recap the history of this engine: We suspect that we had an ignition system failure (either cross-firing or bad module) that caused extreme pre-ignition / detonation on the # 5 piston while running. This took a chunk out of the piston and also blew the head gasket on that side. (pic of the damage: http://imgur.com/AtoBrAA )
Pulled the engine last summer and installed new pistons, rings, rod bearings, oil pump, exhaust manifolds and elbows. Left the cam, crank, and timing set alone. Did a valve job on the heads (including new seals). Replaced ignition coil, ignition sensor in distributor, rotor and cap, plugs and plug wires. New fuel pump, fuel lines, water separator, and carburetor later on once RPM problem became apparent.
We have at least 25 hrs on the engine since the rebuild at this point, and we are just about out of ideas. I've run a bore-scope camera through the outdrive exhaust passages and up through the Y-pipe to ensure there are no blockages. We even removed the exhaust flappers in the Y pipe just to be sure it wasn't a back-pressure issue. No help.
The rest of the boat is essentially un-changed. Same outdrive (Bravo 3 with 26" props), same boat load-out, no hull fouling, no water-logging, weight unchanged. (It lives on a trailer.) And like I mentioned above, every once in a great while it *will* rev up to around 4000 or so, usually on the very first run-up of the day. But never more than once, and even then only infrequently.
And it really should get to 4600 (like it did before the rebuild).
I'm convinced that the inadequate fuel flow through the secondaries is a problem, but I believe this is a SYMPTOM of a bigger problem. I don't think the carb or the fuel system is the culprit, because it did the same thing with all the old fuel system parts, and now with everything new it's doing the exact same thing. Which is why I'm starting to focus on the manifold vacuum, but that just raises more questions...
Sure hope someone here has some advice, because I just don't know what else to try...
Adam
EDIT: Forgot to add that base ignition timing is set at 8 degrees BTDC and the electronic timing advance is functioning. (Verified with a timing light while operating under load). Getting up to 20 degrees of advance beyond base, which appears to be correct.
EDIT2: For posterity, I'm adding this addendum: The ultimate solution was to replace the ignition control module. It was failing to properly advance the timing when the engine was throttled up under load. This was hard to see, as I never kept the timing light on for more than a few seconds during early troubleshooting and the module would only fail intermittently. For the complete history, read the thread through to the end where the solution was eventually discovered (after a lot of work).
