I have a Husky garden tractor with kohler 25hp twin 4 stroke. Only run non-ethanol gas, change oil, replace and properly gap plugs, and replace air/fuel filters at the end of every season. Engine has 260 total run hours and is 4 years old.
Recently just started running intermittently rough. Can be cruising along mowing the field and engine starts losing RPM, tractor speed decreases and engine surges occasionally to full RPM then immediately back to rough lower RPM. Only thing that fixes it for about 10-15 minutes is pulling full choke for about 15-30 seconds, let the engine run at as full RPM it can develop with full choke, belching clouds of black smoke, then push choke back in and good to go for the next 10-15 minutes.
I'm thinking this points to a carb issue, but what? Maybe choke is somehow changing positions? Engine manual references a Smart Choke system, but not sure if that is standard, or optional. Could be the culprit? Choke cable has good friction, all choke linkage is clean and lubricated and securely connected. Tractor is stored inside climate controlled shop and cleaned after every use where dirt/debris accumulate so corrosion is not a problem anywhere on the outside of the carb/engine.
Any ideas for things to check would be greatly appreciated!