sorry this is my 2nd post for this problem but i didnt think to add more info about the problem and what has been done. im working on 3 lawnboy push mowers dura force 6.5 hp. model # are 10247,10247 and 10324. all 3 surge. i have replaced spark plugs cleaned air cleaners on all 3. on one of them i cleaned the carb 2 times with no change. then i replaced the complete carb assy. still no change. one person said it sounded like a lean mixture. very good info that im printed and filed. but one other thing they all do is leak fuel from the muffler (black looking). so im thinking its a rich mixture because of the black fuel and smoke coming from the muffler. any ideas or thought would be great.