Hy cowinthefog, I was taught that the only way a gas engine could run on after the ignition was turned off was because there were glowing carbon deposits in the cylinder and the engine could still get oxygen past the throttle plate. We can't do much about the deposits but we can control the oxygen by setting the carb up to run with the smallest amount of throttle opening possible at idle. It also sounds like your float level is high, or maybe it's getting gas logged. Hope this helps.
