89 80hp low compression missfire

[justinf7]

Bought a boat with an 80 hp 89 merc. Said they ran it last year.... ran a cold compression test at 85 90 90. Got it home and it started right up. Seemed a little rough but I'm used to my 6 cylinder 135. Fogged for a bit with seafoam. Then tested compression. 85 95 95. Top cylinder weak. Pulled plug wire at fast idle and it didn't make much difference on that plug. It did on the other two. I just got done completely decarbing it with seafoam and sprayed a bunch in the cylinder. Im planning on letting it soak over night and checking it. Wonder if my carb needs cleaned on that cylinder?? Getting spark. Just not firing as well. starts right up. Thought weak compression would be hard to start. Any thoughts?

 

[TD_Maker]

Re: 89 80hp low compression missfire

Check the wiring from your stator and trigger to the switch box. Merc motors of this vintage has wiring issues, and the wires connected to the switch box could very well be rotted. Could also be a bad coil. Start with the stator wiring, to the trigger, to the switch box, then to the coils. The compression numbers are a little low, but at least they are close. The compression numbers may get better once the rings get used a little.

 

[JB]

Re: 89 80hp low compression missfire

Your compression is fine and it is not your problem.

 

[justinf7]

Re: 89 80hp low compression missfire

I will check wiring etc. Can I switch coils to eliminate that? (From one cylinder to the other that is). When I pull the plug wire I can hear the spark clicking. The motor rpm drops a little, just not as much as the others. Can it just be the carbs a little gummed up? I REALLY appreciate the help guys!!!! Blessings to all of you

 

[justinf7]

Re: 89 80hp low compression missfire

I did swap plugs from one to the other. No difference. That eliminates plugs. Can I get a spark tester and see if the coil is weak? Just to mention. .. when I sprayed deep creap in the carbs the weak cylinder didn't bog down the motor as much as the others.

 

[TD_Maker]

Re: 89 80hp low compression missfire

You can get a cheap spark tester at any auto parts store. Maybe try and swap the coils from one cylinder to another and see if the problem follows. Again, don't discount the rotten wire issue. Upon inspection, I actually found bare wires (without insulation) and some wires which turned to powder in my hands. It is a very common issue with the older mercs of the mid to late 80's.
I will bet the misfire is electrical and not necessarily carb related.

 

[justinf7]

Re: 89 80hp low compression missfire

Ok. I will check into the wiring. I took it out to a no wake lake this weekend to fish with the kids. The thing starts like it's new. Seems to run and idle fine. Just need to take it out on a lake where I can run it.