I‘be been doing a mini resto on this ‘79 DT100 Going through systems one strange issue at a time. Pin holes in the tank, broken condenser wire, plugged carb, leaking oil tank and more. It’s finally running and on first kick each time but smoking pretty bad. I disconnected the oil tank and went with premix. Thought it may be build up from sitting but fully warmed up still smoking. Then I removed the dipstick to see if the bottom end was overfilled while running. The rpms increased substantially with the dipstick out and normalized when I reinstalled it. Clearly This is introducing air into the chamber. Would it be bad rings or something in the chamber below the piston? I’ve done lots of work on these but never split a case. I recall something about fuel and air mixing below the piston but maybe that’s only on some designs. Any advice would be welcomed. 

Gona have to split the case. The gear box should be totaly sealed from the crank case.

Shouldn't that just be a crank seal, and granted I don't know the later DT100 too well but most of the Yamaha's you can remove the clutch and replace the crank seal w/o splitting the case.

At first I was like “oh S@&T” I’m going to have to split the case and, maybe, never get humpty back together again but the second option would be terrific if that’s possible. Hope there is someone out there who can verify. I’ll tackle it the hard way if that’s the only route. 

Yea on second thought it is most likely just the seal on the crank 

No17 in the pic. Pretty easy to do once the cover is off. Clutch off as well unless you can get at it depending--may not get the crank gear past the clutch basket. Keep in mind that 'if' that is the cause--& sure sounds conclusive--that it'll likely still smoke the village out unless you burn out the pipe & go for a 100 mile run. The oil soaks into the noise deadening packing along the inner walls of the pipe so smoulders away merrily. Kick star might be a fiddle to assemble if ya not familiar with how it works & the clip thingy that tensions the gear to make it move out & engage each kick needs locating in its guiding slot if the shaft comes out as the case comes off. Not a biggy but just traps to catch you out.
www.partzilla.com/catalog/yamaha/motorcy...0f/crankshaft-piston

Don't forget to replace #18 too. It's just an O ring but is a seal too. You have a collar that fits into the main seal ( Crank ) over the shaft but the O ring seals the inside circumference of the collar to the crank .