2011 DC Classic 3372HP with Generac 33hp OHV1 V-twin gas 200+ hours. Engine pops and stutters at idle and while running without pto engaged. Seems to run better with pto engaged at higher rpm's. Also backfires when you shut it down from idle. Have checked the obvious and performed complete tune-up but still not luck solving the issue. Any ideas/suggestions would be greatly appreciated. Thanks