$ stroke engine needs
Air
Fuel
Spark ( at the proper time)
Compression
Air is a given. fuel is a given if you dump it straight in the carb. Spark timing is correct if the key is good. That only leaves compression. causes of bad compression are head gasket, worn valve seats, stuck open valve (chunk of carbon in the seat is one possible cause). Do a compression test. you can also put compressed air in the spark plug hole and see where you hear it coming out ( carb, exhaust, dipstick tube, other...). Only other things I can think of is cam to crank timing is off. someone would have had to take it apart to screw that up, or a intake valve not opening (worn Tappet, or bent pushrod. Is this a flathead or OHV engine?
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