Working on a 17.5 HP kawasaki engine on a JD lawn tractor. It starts OK and runs OK, tends to bog down in thick grass more than it should. If I shut it down it will not restart until it has rested/cooled for 3-4 hours. It turns over fine and I get raw fuel vapor coming out of the exhaust, suspect a weak ignition coil. After cooling for a few hours it will start, sometimes with a backfire sometimes not. Any one have another thought?