To quote LukeM: you need 4 things for the engine to run: "suck, squeeze, bang, blow".
You have suck (gas) and bang (spark). Have you tested your compression? Any bike needs at least 100 psi to run (on its last legs) and factory spec is around 170 psi.
Maybe a valve is sticking?
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals
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