20 hp intek vtwin 4 cin, Scotts riding lawnmower will not start (Year 2001}?

I have 2 questions
1. Lawnmower would run then surge. I would take off gas cap and mower would run again. What causes that?
2. Scotts riding lawnmower. Was running fine. Engine backfired and now it will not start. Engine gets fire but drags and does not start. Could it be valves out of adjustment? How do you adjust them? Or could it be a float sticking?