1981 Yamaha YZ 80

[Vic]

Every time I change oil (transmisson) clutch oil at first start-up tranny oil transfers to combustion chamber and of course causes big smoke & ruff running for a little while. Filled to spec's 600cc - 750cc

[fasterblaster870]

you have to have a seal blown. there is now way any oil at all should make it into the combustion chamber.

[Kane]

Well, if an idiot was the last one to rebuild the lower block you could have damaged cases. I've come across a couple over the years where, Hypothesis here, someone beat on the case mating surfaces to remove the crank. These dents allowed oil to be sucked into the combustion chamber. The seal on the clutch side should always be inspected first of course.