here is a couple of things that could be causing the problem. It could be the crankcase is leaking into the main shaft(the shaft from the clutch to the rear sprocket) area the gasket for the engine's crankcase is very thin in that area between the crankcase and the main shaft the gasket is only about 1/8" wide.
You will be able to see just how thin the gasket area is on my post on total tear down. But if it is leaking you should also be seeing fuel leaking out the front sprocket area at least I think it should or there is just enough grease in the main shaft area to keep it from leaking out to the front sprocket.
My main guess is your main seal on the crank is shot or blown out it is located under the front small gear on the clutch side. Must be about gone to fill up the clutch area.
If you have the kit supplied puller you can take off the small gear and get a good look at the seal its probably shot One of the vendors sells the seals and 2door is the guy to ask which one as I've forgotten but the good news is they are cheap.
The seal in the gear side is easy to replace and find the seal on the mag side is the one hard to find that vendor does sell both though.
I'll go ahead and put the link here
http://motorbicycling.com/f39/bicycle-engine-total-tear-down-391.html
see if this will help I'm banking on it being the seal leaking.
