When you say “checked the compression”, was it with a gauge, or just by feeling the flywheel bounce ? In my experience, not much under 50 lights up or runs well.
If it is Ok , then you’re left with fuel/air mix and supply, are the rods connected to the correct side of the crank and if the rods are in the right place are the pistons upside down on the rods.
Carb lever needs to be on “S”, mixing needle about 1 to 1 1/2″ turns from lightly seated.
Spark advance lever should be just to the right of the centerline of the crankshafts whilst you are sitting facing aft.
If you have decent (60+) compression and the innards aren’t all scrambled, it becomes a matter of making small changes one at a time to the carb lever, mixing needle and advance lever.
Oh, and fresh fuel, 8:1 ratio.
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