I have one that ran flawless for a year and a half. Built a new motor to put on dads so we took it off and put it on the shelf a few weeks. We robbed the carb off it to put on dad's new motor but other than that nothing has changed. Put the motor on my brothers mower with a different carb and now the motor won't run for nothing, it runs but surges and spits and sputters. Only thing different is the carb and my brothers mower has a stubby exhaust pipe (12"-14") compared to a full exhaust on dads. Anybody have any ideas?? I am putting a full length exhaust on it tomorrow evening but still not sold on that being the problem. Could it be a head gasket? Doesn't smoke... HELP!