Well the o ring could cause it, you would be blowing exhaust into the crank case. Also for the block off plates I have found that it's best to uses the factor o ring seal, I have used only them taken off the reed valve plate and I have also used it with the plate with the reeds and screws removed. The other worse case senerio that can cause it is rings not sealing and a lot of blow by. The hose is not pinched or something going into the airbox is it? Also another thing to consider on the pair is it should not be getting crank case pressure, only exhaust. So those two things should not be related unless there is a bad seal in between them.
1974 Yamaha RD350
1971 Honda CB500