My ‘99 EZGO TXT runs fine when accelerating or traveling at mid-throttle and higher speeds. But when on low throttle and coasting it is backfiring. I have cleaned and adjusted the carburetor. I had the engine rebuilt by a well known rebuilder and there shouldn’t be any need to adjust valves or belts. A visual inspection of the throttle plate shows that, at rest, the plate is totally closed. Any help?