The main wheel balance related problems on the 140's are the rears on the V8's and V12's. Because of the weight of the vehicle and the available torque, the tires can spin on the rims, making them go out of balance on the rear. Trick is to either NOT use mounting compounds when mounting the tires, or wiping off all the compound before inflating the tires. Shouldn'y be a concern on your 6 cyl though. Maybe just a bad balance job?