Sounds like you have a compression ring that hasn't seated properly, ot the oil mix is too rich. BTW, it's normal for one side to run a little leaner that the other due to the design of the carb and reed valve. I would consider getting in touch with Gene at BME and checking to see if you could lean out the oil mix. 50-1 may be too rich for the engine, causing loading up of the one plug. Excessive oil would also delay the seating of the rings. You probably don't have a serious problem that requires the return of the engine, but again, a call to Gene would be in order to determine that.