I'm trying to resurrect a 1977 Honda Express II. It runs, but will intermittently start blowing oil out the transmission case vent. It may go for days without this happening, but when it does, it pretty much pumps out the case and makes a hell of a mess. The manual specifies 10W-40 motor oil to fill the case, and I fill it only to the bottom edge of the fill hole, as instructed.
I haven't had the thing apart yet for fear I'll ruin a seal or gasket I won't be able to replace. I'm thinking maybe crankcase gasses getting past the seal between the crankcase and the transmission. Any ideas before I tear it down?