QuestionHi Bill, I have a Honda Cb250 and the compression on both cylinders
showed 120 PSI, could this amount of compression cause it to be blowing smoke
through the breather tube from the head? Thanks. David.
AnswerDavid, they didn't sell CB250s over here and I am not sure if yours is the 68-73 or the 74-77 type or something else.
Excessive fumes coming out through the breather tube generally indicate either worn rings or rings that have gotten stuck in the piston ring lands from sitting for extended periods. Stock compression should be up around 150-175psi with throttle held wide open. CV type carb tend to block air flow through the intakes, so sometimes numbers are lower than the stock specs, taken with probably no carbs mounted or some that have the slides wedged wide open and butterflies open, too.
The other thing that can cause even, but low, compression readings is if the cam timing is off one tooth. The engines don't perform well and they heat up causing potential piston seizures, especially when the ignition timing is off track.
Sounds like your bike is in need of a top end repair, with new overbore pistons and rings, plus fresh gaskets and seals. New valve stem seals and valve seat cleanup should bring it back to original performance specifications.
Bill Silver
