Motorcycle Repair: Gas in oil of CB900 Custom, rich fuel mixture, piston rings

Question
Do you have any ideas on what might be allowing fuel to enter the gearcase. 1981 CB900 Custom. Thanks...Joe

Answer
Fuel can get into the crankcase in a few ways...

Carburetor float valves failing to seal, allowing the carburetors to overflow into an open cylinder's intake valve. That can rarely happen if the bike is left parked on the side stand.

The automatic petcock shut-off valve diaphragm could fail and fuel can leak back through the diaphragm tear and into the #2 carburetor vacuum hose.

Carburetor jets main fuel metering airways are blocked, causing over-rich fuel mixture which dilutes the oil film on the cylinder bores.

Choke cable out of adjustment, causing rich mixtures, as above.

Poor ring sealing will allow more than average fuel mixture by-products past the piston rings and down into the crankcase.

Plugged air filter causing over-rich fuel mixture, as above.

Those are the most likely causes, but there are more possibilities.

Bill Silver