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95 Mercruiser 5.7 Misfire


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Hi hoping someone may be able to offer assistance here. We have a 95 Response with 5.7 Mercruiser Ski engine. This is pre -Vortec with Thunderbolt IV ignition and 4bbl Weber carb. 800 hours in fresh water and engine has never been dismantled.

Last season it started to develop a misfire and run rough between 1000 and 2500rpms. This only shows up when the engine warms up and after maybe 15mins running. Starts and idles fine and also runs on all 8 above approx 26-2800rpm.

We started going thru and so far have replaced the following, all with Merc parts- cap, rotor, leads, plugs, coil, trigger module inside distributor. No improvement. Put new Sierra gasket kit in carb. Starts and idles better following carb rebuild, but misfire still there. The only ignition part I haven’t yet replaced is the TBIV module on the outside of the distributor. They look to be obsolete now.

When it is misfiring, a tiny amount of choke will bog it down, so I am figuring it is not starving for fuel. You can hear it cackling in the intake if you remove the air filter. 

Compressions hot are all fairly even at approx 160psi. 

I am wondering if maybe the TB module is advancing the timing too early and it clears up at higher rpm because it is ok with the advanced timing. Also wondering about intake manifold gaskets or if the carb is worn and jets may be allowing too much fuel thru. When you are skiing behind the boat the exhaust does smell a little rich.

Has anyone had a similar problem and resolved it?

 Thanks in advance.

Andy

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Forgot to mention new trigger module has a third wire which grounds to stud on block. I also made a heavy ground wire from the TB module to engine block. Sadly, no improvement....

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@AndyL-  One ignition component not changed are the plug wires, you might want to check the resistance of them to see if one is bad, you can also run the engine in the dark and see if any arcing exists.  Also, a coil will tend to fail at elevated temperatures and perhaps due to a fuel calibration curve where the midrange rpm happens to tax the spark a bit more than once past it seems a potential.  I would also do a leak down test, you mention the intake cackling and depending on the specifics, that actually could be a head gasket failure which is not uncommon and happens between the two center cylinders.  You can fool it with a compression test if it is in the early stage of failure hence my suggestion.  The Weber carb has a variety of fuel calibration circuits, basically three steps:  idle, midrange and WOT.  One of the elements of the circuits could be off and thus impacting just the midrange and in this case would probably be some debris causing an issue.

You can check your timing curve with a timing light, idle timing should be ~ 8 degrees (although I like a bit more advance there) and max is 32-34 degrees BTDC at 3,750 rpm .  It is a linear advance from 750 > 3,750 and is a 24 degree advance from initial point.  The module is V8-24 or at least that is the one on my Thundervolt IV and going on that assumption.  There are other modules that range from 20 to 24 degree advance and have different advance curves.  Hopefully, that helps for some more troubleshooting.

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Great thank you for the tips. We have replaced the plug wires- our NZ term is leads..!! Should have used the correct term.

The head gasket is a nice idea. Will bring a timing light to the lake next trip and check timing, otherwise it may be that the heads come off. One local guy mentioned that he does not like that Weber carb and has more success with some Asian Holley copy unit, but I do not know what brand. Wonder if I should also try some new jets in the carb. I will check with my son who rebuilt it, but I do not think the Sierra kit had jets.

Aporeciate your assistance. Thanks. Andy

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4 minutes ago, AndyL said:

Great thank you for the tips. We have replaced the plug wires- our NZ term is leads..!! Should have used the correct term.

The head gasket is a nice idea. Will bring a timing light to the lake next trip and check timing, otherwise it may be that the heads come off. One local guy mentioned that he does not like that Weber carb and has more success with some Asian Holley copy unit, but I do not know what brand. Wonder if I should also try some new jets in the carb. I will check with my son who rebuilt it, but I do not think the Sierra kit had jets.

Aporeciate your assistance. Thanks. Andy

If he cleaned it with a spray can of cleaner, do it again but use the gallon can of dip cleaner.  That stuff is a whole lot stronger than the spray cans.  Also use compressed air to clear the passages and orifices.

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Good will go over carb again. One thing I recall is that this problem showed up last Summer just after the engine had sat for the Winter. I am starting to think maybe the head gasket has finally partly corroded thru.....

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Does anyone know which would be the correct version Mercruiser service manual for this engine please?

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I read too quickly, had I mentally been astute I would have noticed when you lifted the bonnet you changed the leads :lol:.

The Weber carb is actually very good, it is the same as a Carter AFB or an Edelbrock.  It actually has less chance to leak fuel compared to a Holley.  It is very simple and easy to rebuild.

Edited by Woodski
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Yes funny. We are a bit English here but I do enough with US cars and boats that I should have known about the wires...

Thanks for the advice on the Weber- I will persevere with it. We did not have a manual when we rebuilt it but do now. I see there are instructions that we should have seen beforehand. Will check it again and also do a leak down test.

Appreciate the advice.

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