Originally Posted by KayAitch
Oh, I haven't been intentionally keeping the revs particularly low at all. I've been winding it up to say 70% of redline (just a guess, no taco) on my running-in rides.
This 'rpm ceiling' is occurring straight out of the driveway for the first 2 or 3 accelerations in 1st and 2nd so maybe that is just loading up from warming up? I do blip the throttle a bit. Perhaps once warm it should be given some heftier twists to clean it up?
Sounds like you have the venting issue on the run. As far as the seals are concerned, due to age the seals may be hard and leaking air. Is this a rotary valve engine with the carb inside the engine case? If so, that may be another place for an air leak. The crankcase sealing can be leak-down tested, but I have no way to tell you the right way to do that. To change the seals (if they are even leaking) you have to split the cases. The loading up issue should pretty much clean out once up to operating temp. It may come back a bit if the engine is "lugged" at low RPM but should clean out again with a twist of the wrist. Gotta love the little old 2 smokes.