I agree with fixer on the timing key. It is interesting that the seafoam helped however. If it was all timing it should have had no effect on it. Have you tried putting a carb repair kit in it? This may be another way to try but I would check the timing key first. It is located in a slot between the flywheel and the top crankshaft. You have to take off the top shroud and metal start shroud. you will see the flywheel then and you may have to at least take off the starter cup and flywheel nut. then you should be able to see if it is sheared. Let me know if you have any more questions I will try to help you through it.