1990 Johnson GT150, compression on starboard side 85,90,95-port side 100,95,100. My mechanic says that compression is to low and that if I run it easy it should make it through the season but I'll need to rebuild next winter. From what I've read this isn't that low of compression for that motor. Should I expect trouble down the road? Thanks for the help
Have you ever decarbed it? If not I would decarb it and see if the Starboard side comes up some. You might check head temps, sounds like the sb side has been running cooler than the port.
I have a 1985 Johnson 150GT and my compression is 85-90 on all cylinders. I had also seen someware that the older GT motors had lower compression than others but never got a real answer from anyone to verifi this. When I talked to a tech he said it was fine as long as it wasn't more than 10% between cylinders. He is a youger guy so he wasn.t sure on the compression thing. Wish one of the tech guys on this site knew about the lower compressin on these motors. I know that my 150 still seems to get up an go pretty well.
Lots of variables when checking compression numbers. Best thing is comparison among cylinders not a number on paper. If not done, a good decarb is in order I'd say. I'd almost bet your numbers go up.
J150GLERV
Guess you are on to something because I was a auto tech all my life but I know that the newer cars now use percentage between cylinders instead of a regular compression reading. Guess I'm just old school and got to look at my 150 in a differnt light. I plan on doing a good decarb this spring when I wake up my boat, after the carpet job gets finished. Now if only it would warm up in Wisc.
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