This one is very frustrating. GP25KH gasoline only. Installed new engine due to low compression. Bought through a cat dealer. Ser#AT17B95880. Set timing at 4 degrees btdc. Had carb rebuilt. Machine is hard starting, you have to pump it several times to get it started. It idles erratically, not smooth. Carb idle air mixture screw about a turn and a quarter out. It runs horrible. Very slow idling up when you hit throttle. Backfires randomly through exhaust and air filter/ carb. Backfired once after it was shut off. I'm at a complete loss on this one. Any help is greatly appreciated.