My mower is 3 years old with 75 hrs on it. Up until now, the engine has run great. Then all of a sudden, the engine starts surging at all throttle positions. Did everything I could to solve the problem. I eventually purchased an aftermarket carburetor and installed ot. runs great. No surges. Now I have a new issue. If I turn off the engine, it will not start. I can spray carb cleaner in the carb while turning the engine over, and after about 10 shots the engine will stay running perfectly. I have tried to adjust the carburetor, no luck. What am I missing?