Sounds like a lean backfire. With the engine shut off and the air cleaner removed and wearing safety glasses look down the throat of the carb and push the accelerator. Do you see a stream of gas shooting into each front barrel? If not the accelerator pump has died. Could be just gummed up from setting. Fuel turns to varnish like gunk when it evaporates. Most likely needs taken off, taken apart, cleaned good and new gaskets installed. If it is indeed backfiring through the carb it could catch on fire if driven like that.