Could be a number of things air leak,carbs, or ignition..
U can try the carb cleaner trick for a air leak..spray it where U think it's leaking from..
If the plugs looks ok than rule out jetting...
Did U have the flywheel off for any reason or messed with the pick-up coil..
If so, it should be gapped between 18 to 25 thous. and check the gapped on the plugs also...