Well, I fixed one problem and created another. My Super A (12 volt) had become really hard to start so I checked the coil and it was bad. I replaced it and the tractor started much better. Well, I says to myself, let's go one better! I replaced the plugs, plug wires, points, condenser, and rotor button. It started and ran fine. I only let it run a few minutes, though. I shut her off to "fine tune" and, then when I strted back up the engine was really rough. It ran OK at idle, but when I advanced the throttle the tractor spits, sputters, and backfires through the carb. I have checked everything (I think) even to the point of draining all the gas and putting fresh in. What can I do?