I never had a problem like this and I'm stumped. First thing in the morning the mower engaged the starter when I turned the key but it wouldn't start. It wouldn't spin over. Acted like the battery was dead or really weak. I charged the battery but still no start. Then I removed the plug and it spun over fine. Replaced the plug and it started fine. I used it all afternoon with no starting issues. Next morning the same problem, turn the key but just wouldn't spin over. Figured it must be the battery not holding a charge. It was an old battery so I assumed it was a battery issue. I replaced the battery and same issue. It just wouldn't spin over. I removed the plug and it spun over great. Put the plug back in and it started fine. Now I'm thinking maybe a starter issue but removing and reinstalling plug to get it started has me confused. Anyone else run into an issue like this. Motor is a Briggs 10.5 Intek I believe, on a Troy Built rear engine rider.