Howdy, hope someone has experience with this. A very good widow friend bought a new 2021 Mule with the single cylinder engine. She hasn't got a tank of fuel through it and it started to run rough and had the dealer come out and pick it up. They told her the plug was fouled and put a new plug it and brought it back. It ran fine for about 5 minutes and started to act up again. She lives out in the toolies and asked me to look at it. I pulled the plug and it was fouled with black soot, I cleaned it and put it back in, but no joy, I also pulled the air cleaner to look at it and it was clean and unobstructed. The plug didn't have fuel on it, just black carbon like it was running very rich. If I had to guess I'd say too much fuel pressure or bad fuel mapping. It doesn't get any better with full throttle. Dealer told her to let it idle for 15 minutes which I've never heard of before.