my terramite with a Briggs and stratton 16 hp cast iron engine has been converted to run on a car coil. The engine has been running great, the only problem is that it backfires sometimes when shutting it off. Today i was clearing some land and it started running rough all of a sudden, cutting out under load, backfiring though the exhaust. I got it home and started playing around with it. It fires up no problem and runs like a top at idle. when I throttle it up it spits and sputters and backfires. It cuts out if I try to move it when it is at a higher idle. I have tired to adjust the carb with little to no improvement. I changed the fuel filter, gas looks clean. I have recently went through the entire engine, new head gasket, lapped an adjusted the valves, cleaned the carb, new plug and plug wire, throttle springs, etc. not sure what is would cause it to all of a sudden start running bad. Any suggestions?