compression on a 90hp johnny

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alanR1

Petty Officer 2nd Class
Joined
Apr 13, 2003
Messages
109
I recently brought back to life a 90hp v4 johnson outboard. it is a 1982. it has been bored out with oversized pistons. i rebuilt the water pump, thermostats, carbs, etc. it was seized when i got it and i broke it loose and cleaned it out. just got it started and ran it at idle for about a half hour. checked compression and all 4 cylinders were exactly the same at 86 psi. is that ok, it revs out and runs strong. haven't had it in the water yet though. any suggestions on how it should be..
 
D

DMR Mechanic

Cadet
Joined
Nov 18, 2003
Messages
9
Re: compression on a 90hp johnny

There is no set psi that I know of, just that there should no be more than 15psi difference between cyl. If the motor is not broke in yet, that may account for the somewhat low compression. I would run the motor under normal conditions for a while and then recheck the compression. I think it's OK.
 
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alanR1

Petty Officer 2nd Class
Joined
Apr 13, 2003
Messages
109
Re: compression on a 90hp johnny

sounds right, the motor sat for at least 2 years frozen so I imagine the compression may rise after the rings get a chance to clean up the cylinder walls. Thanks for the reply..
 
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crazy charlie

Vice Admiral
Joined
May 22, 2003
Messages
5,581
Re: compression on a 90hp johnny

I would run with a break in ratio of outboard oil and also add a can of seafoam to the fuel.If you keep the seafoam at a richer ratio than recommended you will get a good cleanup.Charlie
 
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