Suzuki DF140 Missing at low RPM's
I have a 2007 df140 that is missing at low rpms once I hit anywhere from 1500-2000rpm it kicks in and runs on all 4 cylinders. I can then almost ease off the throttle to idle with out it missing again. At higher rpms it runs perfectly... Have pulled the plugs and put new ones in and it doesn't change. When the engine is cold it doesn't seem to happen as much. When I pull the #3 plug it looks like the rest from the threads and gasket right to the electrode.
But where the porcelain meets the shell it almost looks rusty... Maybe it is getting hot?? I tried iridium plugs and it made no difference, I also tried swapping the coils with my other motor as I run with twin df140's.
I am now thinking maybe a small crack in the head anyone have any other ideas?
Anything else I should trouble shoot?