Misfire

Larplaz

Recruit
Joined
Jul 7, 2020
Messages
2
I have a 1989 Ebbtide with a 5.7 liter Mercruiser and Alpha One outdrive. Starting this season, when we launch the boat and run for 20 minutes or so, it runs like new (4400 rpms & 54 mph). After running, we stop and anchor for 1 to 1 1/2 hours. Restarts fine and when we throttle up to wide open the boat will start to miss. Max rpms less than 4000. Throttle down to roughly 2000 rpms and misfire goes away.

Have changed plugs, wires, rotor, cap and sensor in distributor. Cylinder compressions from 145 to 155. Fuel pressure from 4 - 7. Carburetor rebuilt a few years ago.

I am convinced it is heat related, as when anchored, the heat builds up in the engine compartment. Again, it runs fine/normal when first started up and run. I'm thinking electrical/ignition.

Thoughts/suggestions?
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
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Jul 23, 2011
Messages
49,967
First, Welcome aboard

Second, I would go back to the carb. sounds like a lean miss
 

Larplaz

Recruit
Joined
Jul 7, 2020
Messages
2
Thanks Scott. I'm not sure I agree it's the carburetor, in that it runs fine/perfect when first launched and run for 20 - 30 minutes.

The "missing" only occurs after it has set for a while (when anchored & floating) and then started and run. During the "anchoring", no air is flowing through the engine compartment and the residual heat is just building up in the engine compartment. I think if it was the carb, it would present itself in that first 20 - 30 minutes.

My thought is that some part of the ignition system is being stressed and/or affected by the heat build up. Unfortunately, I've been unable to further water test it by maybe opening the compartment and letting it thoroughly cool after it has acted up and see if that resolves it.

It has been in the shop twice now and is back there now. They, thus far, have been unable to replicate the problem in the shop.

I've thought possibly the coil but, logic tells me that a coil is generally bad or good - not intermittent. I don't want to swap out the $600+ engine control module.

Again - thanks for your thoughts.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
49,967
its never the coil, except maybe 3 out of a million times

its not the engine control module

go back to the carb. spend $20 on incidentals and $35 on a carb kit.

you have crud floating around in your fuel bowl.

while at it, inspect teh contents of your fuel filter
 
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