[storm9c1]

Folks already gave good advice. But after you pulled the cap, did you verify that the rotor is spinning and is not loose? I had the exact same thing happen once. Screws broke off on the rotor (very strange), so the rotor was just laying on the metal plate with the plate spinning underneath it when cranking the engine. At first glance, it seemed fine until I could lift the rotor right off! There was also quite a bit of plastic shavings underneath like the screws backed out slowly and then just sheered. After replacing the rotor, I was back on the road. The symptoms were the same. I'd crank it and eventually get a backfire because the rotor was in the right position for one cylinder with each revolution, and that would ignite the built up fuel in one cylinder and then in the exhaust. Good luck. Keep us posted. Try some starting fluid to eliminate fuel delivery.