Compression results
 

Did comp. test this morning with a brand new screw in type tester. Had to do it with the engine cold, I can't start it. I got 120 lbs. on all three starboard side cyl. Had 100 lbs. on all three port side cyl. Also this is the 3rd time I found black buildup on the lower starboard side plug. I just changed the plugs last week, only ran motor about 2 hours since. I will try seperate fuel, I think those numbers were like that since I got boat, I feel yesterday was fuel related, just wanted to check comp. Is there a problem with these comp. numbers ?

I'm scratching my head. Even compression across all cylinders is best. What motor? It should run with 100 lbs of compression. Is it possible that one of the cylinder heads was replaced at some point or maybe surfaced? Either could mean different cc combustion chambers. If that be the case an easy fix would be to have the lower side head cut to make them more even or just find a matching pair. Being 20lbs off from port to starboard seems a little much. All my Mercs and Johnnyrudes are pretty even. +/- 5 lbs or less If memory serves the 90 Johnson only had 100 lbs after break-in with new rings and pistons. That was over a decade ago.. Sharper minds will chime in soon.

I'm with Ridgerunner. Sounds like the motor had a rebuild at some point and either one bank was decked or the head was milled. That is the only thing I can think of that would explain such variation between banks when the variation between the bank is +/- 5%. Keep us posted as to what happens.

Being this is a 60 degree looper, the 120 sounds alot closer than the 100 reading. I would expect to see around 110-115 out of it. I believe they are finger port engines which makes them slightly higher compression. 20 pounds difference from one side to the other is a bit excessive, but doesn't really surprise me since you've said this engine was recently rebuilt.

Any ideas on what a 93 200 Yamaha 2-stroke should be at for comp. When I did mine it was 115-120 on all 6 cylinders. It was a little rough the first time I started it and let her idle tell warm after that all good. She run like a top at all speeds with and without a load.

I was going to say 110, so it sounds like yours is in good shape.

Good to hear now to keep it that way. Looks like the guy who owned befor me took pride in his toys and had some $$$. everything he did to her wasn't cheep stuff of poorly done installation jobs.
Thanks again for the info.

Yes I purchased the engine newly rebuilt, in Sep, 2010 I was told that when he got the engine in off of another boat he opened it up there was nothing wrong, he changed the crank and rebuilt it anyway, but I have a problem trusting this guy having the problems I have had so far, I know he just slapped used parts on it to get it working, it has been running O. K. felt like I was a little week on power though. My past posts have told about my RPM / MPH issues. I don't really know what to do next ? Thanks for everybodys responses.

normally there are two different head gaskets available for Johnrudes, ones thicker than the other to drop compression for commercial use, some one could have replaced a head gasket at some point

never mind, I see thats its a 60 degree motor, I think they use orings. Check the casting numbers on the head and make sure they match