First check it for intake or exhaust leaks, which could possibly cause that. Especially a hole or rip in the rubber manifold hose which connects the carb to the engine.
Then, tell us what jets it has in it, and what altitude you are at, and what air filter and exhaust system you are using, and maybe we can help.
It's not '64 500. There were none. It might be a '64 frame, but it has a 1990s engine in it.