On a JD GX255 Will not start.Backfires thru the carb. Things we checked Turns over good Spark both coils good Flywheel key good Compression 95 both cylinders Fuel shutoff valve good Fuel pump good Fuel to cylinders yes Valve clear .005 Took off intake and inspected intake valves.Both are opening and closing. With intake vales closed filled both intake chambers with gas.No gas leaked out in 24 hours. Is there anyway a gear on the cams or crank could of slipped out of time? Can Somebody please help me!!!!!!!!