Ser# CG470L-0204-9636KF. We sent this unit into the shop to have the starter and flywheel replaced which means they had to pull the engine. When we got the unit back it was back firing again, sometimes difficult to start and the front differential would start smoking after hard use. Could the backfiring and starting problems both be caused by a bad ground? Could the smoking differential be caused by something done incorrectly when the flywheel was replaced?