Pull the head and check the intake is seating and sealing properly. Had one where the valve looked good but gave random backfire in the carb. New valve and a lapping job fixed it.
There are 2 different wheels that look identical but the metal gears are not the same. Seen this problem when cust got new wheels off amazon and the gears barely mesh. Not the correct wheels but "looked" right New OEM wheels and drive gears fixed it.