Motorcycle Repair: oil leak 1983 yamaha xt200, drive sprocket, vent hose

Question
Chris,
My question concerns my bike pumping oil out of the crank into the air box through the vent. I changed the seal behind the drive sprocket which was dry rotted and leaking then topped off the oil to the correct level in the sight glass, but now the bike builds up pressure and pumps oil into the airbox. what could fix this.

Answer
Hi Eric.
 What condition is the air filter in?  Is it dirty?  If so, then the air flow could be blocked and the carbs will pull from the only remaining source; the crankcase via the vent hose.

Start with that and see what happens.
Good luck.
FALCON