1000 cfm on 470 ci ??? Seem a bit large IMO but,

Keep it simple, IE: In order for it to gain RPM it has to be getting more air or the timing has to be advancing.

Try it with the throttle linkage and vacuum advance (if your using it) disconnected . It would also be a good thing to verify your timing mark is stationary while this is happening

Also make sure your secondary blades are not moving (Long shot).

Check the gasket and screws between the baseplate and main body. I've been bit by that more than once

If the above checks good I'll agree there appears to be a leak between the base of the carb / intake or internal to the carb. If using one of the soft paper gaskets you should be able to see the imprint of the baseplate on it.

that'll keep you busy for a bit keep us posted