Hmmmm. I might start with a cold compression test and then another done after you have warmed up the engine. Some carbon under a valve seat might undo you. Do you hear any chuffing from the exhaust? Have you tested the fuel pressure delivered by the fuel pump? If the mechanical is hanging up you might idle OK but have fuel loss at speed. Are you sure that there is no exhaust obstruction or obstruction in the air into the carb? I know a guy whose bug sucked up a piece of a plastic bag ( long story) and diagnosing the problem took forever because you just don't suspect that. Can fuel contamination be a possibility? Lastly, a sticking carb float might starve your mill of gas as might a crimped fuel line. I can guarantee you that it will be the last thing you look for! Dan ( MY72BUG - frozen in Goderich, Ont.)
I'd rather have a partial bottle in front of me than a partial frontal lobotomy.