1997 40 hp mercury low compression

[grasshopper45]

1997 40 hp mercury
It all started with, won't low idle. I have rebuilt the fuel pump and cleaned the carbs. Took a compression check and only has 80 psi. That is on all cylinders and they are all even. The motor really dose not have that many hours on it. Can't really check the timing because it will not idle. Question what is the trigger mechanism about. When I lift the lever to start it will take about 15-20 seconds to go into high idle. Hopefully someone will tell me it could be stuck rings or bad head gasket. Also is selco the best manual as I do not have the manual yet.

 

[Bosunsmate]

Re: 1997 40 hp mercury low compression

It could be a few things incl seized in rings, Run some seafoam through it and see if that improves things.
Trigger tells the coils/powerpack when to fire.
Check you have spark on all cylinders

 

[grasshopper45]

Re: 1997 40 hp mercury low compression

Thank You bosunmate! I have read in this forum there is a Yamaha solution for sticky rings. You put it in the cylinders and spin it. Can not remember where it was. I also forgot to mention, the top carb has only half the vacuum as the bottom one. I will do a better spark test tonight. Is there a resistance test number on the coil I can use. Question is the Selco manual better than the OEM service manual.

 

[Bosunsmate]

Re: 1997 40 hp mercury low compression

Might be the reeds for the top cylinder are stuffed, You might see fuel getting blown back through it.
Or you could have a failing upper seal.
Id see if you can get those compression numbers up first as that may fix the vacuum poroblem.
Not sure about resistance values, just check spark can jump a 7/32 gap for starters and its consistent.

I find the OEM manuals only average, never tried a Selco so it might be better

 

[grasshopper45]

Re: 1997 40 hp mercury low compression

Thanks Bosunmate! I tried the spark test and I really wasn't impressed. Really don't have blow back on the top carb. I will try carb spray under the flywheel, to see if it the RPM goes up. I think you are right on the top seal. The spark test i did was at .050 gap. 7/32 is .218 that will be fun to see if the spark can do it.(wow) I really need to get the manual. Thanks again.