hello there,remove your clutch cover off your motor and then inspect the seal just under your drive gear to see if it leaking,and if it is then that means you have a blown oil seal on your crank shaft of your motor. had a simuler problem on one of my motors before.my oil seal blew out and exhaust gasses where exiting around the primary 10 tooth drive sproket. thats what it sounds like the problem you have there. might be a manufacturing defect in your motor.
p.s. thats on the right side of your motor.