1997 OX 225 2 low cylinders

[tdelotto]

I would appreciate thoughts on the following - did a compression test on this motor and the top 4 cylinders show 120 while the bottom 2 - # 5 and # 6 showed 110 and 115. Thought possible head gaskets so I pulled the heads. The combustion chambers for these 2 were whistle clean while the other 4 shoed carbon on piston tops. And the air box ports under both # 5 and # 6 had a lot of raw fuel in them so thinking this is washing the bottom cylinders. Head gaskets are OK and all cylinders look fine - good cross hatch still , no scores, no piston wear no bearing slop.
Thinking there is a rich fuel condition - winky blinky did not show any codes.
o2 sensor new late 2014. Have not checked injectors but they were cleaned and tested in spring 2015 - all Ok.
Any ideas on a rich fuel condition? Wondering if a fuel pump is leaking fuel into the # 6 cylinder and its crossing over to # 5?

[boscoe99]

Was there any particular reason that you decided to perform a compression check? Was the motor not operating and performing correctly?

Cylinders 5 and 6 in your model motor have a lower compression ratio than do cylinders 1 thru 4. It would be abnormal if cylinders 5 and 6 were the same as, or did not have less, compression than 1 thru 4.

An excessive fuel/air mix would normally result in more carbon build up. Your thinking that excessive fuel in 5 and 6 may be reducing carbon build up is contrary to conventional thinking.

Carbon build up will result in high compression test pressures. If 1 thru 4 have more carbon than do 5 and 6 then that would be another reason for 1 thru 4 to have more compression.

The compression values that you have reported are all well within Yamaha's specifications. I don't see a problem with the rings sealing.

Cylinder 5 is completed sealed and isolated from cylinder 6.

[rodbolt17]

and remember, Yamaha is bank to bank not inline.
you learned a lot about your engine.
blew a few coins on the way.

\we have all done that.