140 chevy Runs great wide open but at 3000 to around 2000 RPM's it back fires

I rebuilt the carb installed a new fuel pump and all fuel lines. Vent is clear I also flush and refilled the fuel tank. I had the same problem before any of this was done. Stumped... I also have to pump the thottle to get it back to 3000 RPM or above. Thanks for your help this is by far the most helpful forum I've ever used. Thanks again.

Welcome aboard !

Check under the dist. cap

If the rotor can flop around, an advance spring has corroded and failed.

IF the rotor can't move against spring pressure, the advance mechanism is stuck from lack of lube.

There could also be cross firing under the cap from moisture.

Another cause could be a lean condition, one of the primary jets are clogged. peer down the carb. but keep your head away to view any disproportionate flow from the venturi's.

If there is junk in there, its only a matter of time before it clogs the idle circuit too, so search for the root cause else you'll re-re-rebuild the carb all summer.

Thank You for responding. I will check all your sugestions and post my findings.

Keep us all posted !