Small Engine Troubleshooting Problems? Ask a Mechanic Now.
Try adjusting the valves! This engine needs valves adjusted occasionally. If valves are too tight, they start to leak as they warm up. Looks like lash should be ,010 inch on intake and exhaust on the FD620 engine. It may not be a sensor or fuel problem! And when engine starts to misfire, the yellow light comes on from exhaust sensor seeing a rich mixture. Worth looking at!
Did you find the problem?
Intermittent problems, always hard to diagnose, but with it blowing back through throttle body, would seem to be spark occurring with valve not fully closed. May be valve sticking. But with it stopping when it is shutoff and restarted that does not fit. That leaves the spark occurring at wrong time. Do you have a spark tester, so you could test the spark on each cylinder when it is acting up? If spark gets erratic on one cylinder, you could have a bad trigger coil. This engine has two triggers, two coils and an igniter.