An exhaust backfire is typically a loud bang out the exhaust pipe, caused from accumulated unburned fuel in the muffler. If it pops once on a hot restart, the muffler was filled with unburned fuel as the engine was coasting down after the ignition was cut. That's what the solenoid on the carb bowl is for, to stop the fuel when the ignition is cut, doesn't always work. It can also happen when a safety switch has intermittently cut the ignition (like the seat switch), then the ignition is restored, the fuel in the muffler ignites.
If random poping out the exhaust while running, that is still unburned fuel in the muffler. If the engine is misfiring, it is a sign of intermittent ignition failure, like a bad plug, weak coil, kill wire shorting to ground. Could also be a vacuum leak, even an exhaust leak drawing air into the muffler enough to cause fuel burning in the exhaust.
Intake backfire is popping back through the carb. This can be more serious, can melt the carb, burn the air filter, etc. It is caused usually be a too lean mixture, vacuum leaks, faulty ignition, valves out of adjustment, exhaust valve not fully opening, or exhaust restriction. Continued running, if severe, can cause the engine to overheat, gall the piston.
On a simple single cylinder engine, there aren't too many places for vacuum leaks. They are usually a result of the carb mount bolts or the intake tube being loose. Some use gaskets, which can suck in, causing a leak. Others use orings which can usually just be tightened up. If a lot of hours on the machine, sometimes the throttle shaft will wear the carb housing, but that is doubtful on your model.
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Today's Featured Article - New Life for an Old Allis - by Tyler Woods. My friend Jon, has an old '39 Allis Chalmers B. He thought it a marginal tractor that had long since served its time. She smoked terribly and never had much power but he couldn't afford another so he was limping along with what he had. Jon's Allis has a small front loader and though it doesn't carry much, it serves his needs. It was the hard starting and low power that made him think it was time to replace the old girl. Jon called me to help him discover why his tractor wouldn't start
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