I just bought a 2007 S40. It starts when cold but has a rough idle. Once it warms up, I have to rev the engine to keep it running. If not, it will quit and is hard to start again.

Apparently the carburetor was cleaned last year and jetted for a pipe by the previous owner. I test rode it before buying it and it was very loud and backfired a lot.

The previous owner put the factory exhaust back on before I bought the bike. I had an experienced mechanic rejet the carb for the factory exhaust and the he recommends checking the valves next.

Anyone else had this issue? How did you fix it?