AboutBill Silver Expertise Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.
Experience I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.
Organizations VJMC (Vintage Japanese Motorcycle Club) of North America
Publications VJMC newsletter, as editor for two years and as contributing editor currently.
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.
