Briggs and Stratton 42A707 surging at idle
Briggs and Stratton 42A707 2238E1 9801145B Engine
Bought a used riding mower last summer with this engine. Seemed to be running fine last summer, when I went to run it this spring it wouldn't maintain a smooth rpm when idling at no load. First thought was bad fuel, so I replaced the fuel with fresh fuel treated with fuel additive, replaced the fuel filter and one of the fuel lines, replaced the air filter, changed the oil and sprayed the carb with carb cleaner. I'm getting good fuel flow from the tank to the carb and the carb seems to be getting adequate fuel. Engine still surging. So I replaced the plugs and it seemed to run fine for a few mowings and then it started surging again. When it surges if I lower the throttle enough it will eventually stall.
I replace the plugs and started troubleshooting again and thought I had a problem with the coil because I was having trouble running the right cylinder (if sitting on tractor facing engine) by itself but I could run the left cylinder, so I put the ignition wire from the left cylinder on the right and was able to run the right cylinder by itself (so i assumed the cylinder/pistons/valves are ok). Couldn't test the right ignition wire on left cylinder because it was to short, but when I tried to test for spark on that ignition wire I couldn't get one. When running on the one cylinder it seemed to stop surging, so I figured I needed a new coil. Put on new coil and engine seemed worse. Had trouble getting it started, especially once the engine was hot. Couldn't get it to run on one cylinder. So I put the old coil back on and it seemed to start better and now I can get either cylinder to run on their respective ignition wire, seem to be getting good spark on both wires, so I'm not sure if there's an ignition problem or I just wasn't testing properly, but the engine is still surging.
At this point I'm not really sure what is causing this. I've removed the carb and started cleaning it, will be getting a carb overhaul kit today and hopefully putting that all back together tonight. But the carb doesn't look dirty and I found nothing stuck in the jet. When the engine was running I could see fuel spraying into the carb, so I'm not really to optimistic that the carb is the problem. It seems to be getting good fuel, so it doesn't appear to be a fuel restriction problem. It seems to be getting good spark, so I'm not sure it's an ignition problem, unless the flywheel key is damaged and the timing is off. At this point I'm wondering if it has something to do with the governor or valves/pistons. Any suggestions?