I have a 30" Ariens rider with an 8HP Tecumseh TVM195. Had a lot of trouble with it last year. Hard starting and ran rough, Rebuilt the carb and it didnt help. Replaced the carb this year, fresh plug, new airfilter. Starts okay, but now, it only runs for about 20 minutes and begins to misfire and wont stay running. What else do I need to look at? Coil? Thanks in advance. Jerry