The idle screws being turned all the way indicates that the carb is running to rich.
Backfire in the exhaust indicates the carb is running to rich dumping in more fuel then can get completely burned before the exhaust stroke. All that unburned fuel is getting ignited in the engine pipe, muffler, tail pipe. That is causing the popping sound.
Edit to add check the timing as well.
Last edited by 32vld; 07/09/2015 1:04 AM.
