Motorcycle Repair: 1985 Honda Shadow VT500, blown head gasket, ventilation system

Question
I was working on my bike last fall and decided to let it run for about 10 minutes after working on the cooling fan to determine if it would work. Then when I came out it was real hot and started running funny. I tried driving it around the block the next day and it ran really bad, like a piston wasn't firing or something. I looked over things and found gas in my oil tank. I am thinking it is a blown head gasket, but frankly I have no idea what would cause gas to get to the oil tank. Can't afford to take the bike to a shop and I am pretty good working on cars and am up for a new challenge. With a manual do you think it is economical and possible to fix on my own? Also, any ideas of what would be wrong? Any help would be appreciated.

Answer
Chris, start with the basics...

Pull the spark plugs and see if they are fuel fouled or show signs of coolant or excessive oil on the ends.

Do a compression check and/or coolant pressure test to determine if the head gasket is leaking somewhere. If the cooling fan didn't come on and the coolant got too hot, the gasket can get blown out from the pressure. If you have good compression, then you have more tuning problems than major mechanical issues.

If you are sure that there is gas in the oil, then the only way it can get in there is from a misfiring cylinder, allowing unburnt fuel vapors to collect and get pushed past the rings. Likewise, if the carburetor float valve gets unseated, the fuel can run down the intake port and past the rings into the oil.

The vacuum petcocks on many Hondas can have diaphragm failures, so that you get an air leak, plus straight fuel can get sucked into the intake manifold and down past the rings.

In some rare case, the carbs could overflow into the air box and perhaps  flow fuel backwards into the PCV ventilation system, which vents fumes from the crankcase back to the air box for rebreathing.

Get a good repair manual that covers the bike and study up on how it is all put together. There are free online illustrations at www.bikebandit.com  I would suggest replacing the coolant, the thermostat and rechecking the cooling fan switch in the radiator.

Bill Silver