No burnt valve or timing belt.....it's a 2 stroke. I would check the power pack that controls the coils. There should be an OHM test procedure in your service manual. I had to replace that on mine not too long ago, same symptoms. Same motor also, pretty much. My Carolina Skiff has a Johnson 40, '90 model I think.
You could also check the magneto under the flywheel, sometimes the vibration knocks the pick-up loose and causes a bad reading.