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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.

  • Like 1
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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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  • 5 weeks later...

Update....

Checked timing advance with light- looked good and linear. Dismantled carb again and all passages looked clean. I was suspicious that maybe the step-up pistons were not functioning correctly and were lifting the mixture rods too early, causing rich condition at mid speed. All looked good except that the step-up pistons are a very loose fit in the bore....

Ran a leakdown test and all cylinders were good except No 8. Checked compressions more carefully and No 8 was 15psi less than the others.

Brought boat home from lake and have pulled heads. Found a couple pieces of what appears to be the inner exhaust valve spring from No8 sitting loose beside the spring. Taking heads to a machine shop this week for testing. Bores look good, gaskets looked ok, cannot see any obvious problem with valves on casual inspection.

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If you have a caliper you can measure stem height as an indicator of a bent valve.  The step up needles are a loose fit.

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OK will check valve. Thanks for the tip re step up pistons.

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Posted (edited)

@AndyL - what was the leak down number, that is what is telling you true health.  Compression test can fool you, in the case of a leak, it is the first pump that gives best indication of issue.  If you can hear a fair amount of air passing through the valve, sounds like a bent valve a possibility, but if very little spring pressure due to broken springs it may simply be hanging open a bit.  You can test that by simply pushing on the tip.  There are some decent books on Carter carbs and since same as the Weber worth reading.  Edelbrock carbs are also basically the same.

By the way, beautiful country.

Edited by Woodski
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  • 2 weeks later...

Sorry I did not see your response until just now. Do not remember the actual leak down numbers, but the other 7 cylinders were all even and this one was 30% off from their readings. Have the heads at a machine shop now. They suspect possibly a weak valve spring. Replacing springs, crack testing heads and lapping valves. Hoping to reassemble engine this weekend and maybe a test run following weekend.

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