Leak down test. As cheeta said if it turns it was a most a soft seize. If a stud backed out or the nut c racked on one stud which happened to me it will die. A simple compression tester with a drill start will show a leak at the head if u mix up a dilute spray bottle with johsons baby shampoo. Spray liberally but to ascertain if a seal failed or other huge leak elsewhere I block off exhaust, rely on the Reed's to block off intake , inject air at 7 psi via the spark plugs hole. ( I use a harbor freight fourstroke leak down tester in series with my compressor turned down so less than 10.psi (7 is what I shoot for) and see if it tends to hold it without using additional air. Spray the baby shampoo on base gasket etc. If not holding and leak not at base gasket then u are obliged to uncover the seals and then move the blocking the intake proximal from the Reed's to include the entire intake. And yes if the carb - intake isn't sealed you can miss that. Major pain in ass. I have to do this to my red Hobbit that won't start with the treats puch-hobbit head. This week.