Hello Gang, I have a sick engine on my workplace Exmark commercial zero turn mower. 12 years old with 900 hours. 23 hp Kawasaki FH680V 2 cylinder engine. Air and fuel filter good. Plugs are at 150 hours, look good - maybe should try replacement first? Fuel tanks venting o.k.. Lots of gas in the fuel filter - seems that fuel pump is o.k.? Excellent maintenance and fuel management (E10). I ran about two tanks of Techron infused gas with no improvement. Symptoms: Engine runs rough and seems to lack power. Occasionally pops/backfires when release the blade pto. Go over a BIG bump and occasionally can cough a bit, lose power. Key symptom: Runs better, about 85% o.k. when I have the choke lever at 50%. My hunch is either carburetor or maybe valves (never been adjusted)? Anyone have diagnostic procedure suggestions before I begin parts swapping? What symptoms would be tied to out of adjustment valves? FYI, the carburetor has fixed jets with only idle speed adjustment. Rebuild parts run about $130 (no kit). OEM replacement = $225+. Chinese replacement = $65. Not keen on Chinese carb on such an expensive machine. Thanks for any ideas. I'm pretty small engine savvy - have done rebuilds of entire engines and components. Hate to replace carb and not fix it.