I have an Onan engine 20 hp Performer XTS that the carb was rebuilt and new intake manifold install with new gaskets. Ran fine for about eight hours and now I got it back in the shop with the same problem....surging. It will run fine as long ads you dont engage the mower the it will start surging. shut the deck off and ring it down to idle then slowly back upto wo throttle and it tuns fine. Any assitance will be helpful. Thanks for taking the time to answer this thread.