My 2002 Scotts 1642h lawn tractor stalls sputters chokes & backfires when blades are engaged but will operate at low speed or w choke on and resume running smothly when blades are disengaged. The carb has been cleaned thoroughly, new fuel filter and plug.got any suggestions ?

Check all pulleys and belts, something may need to be lubricated

Your carb may have been cleaned but that doesn't guarantee that it does have a speck of junk in it. I've rebuilt carbs and still had the problem. A $190 new OEM carb was my final solution.