Is the choke open (warm engine) or partially closed (cold engine)? Maybe a low float setting? If the level is low it may take a delayed time for the fuel to pull through the venturi. It might be low due to a weak fuel pump or a clog that is BARELY getting enough fuel to idle and squirt the accelerator pump - only. If everything is stock that car might have a De-Cel valve and it might be getting triggered to bleed air when the throttle is opened. I'm not familiar with those (always gets removed) but others have reported problems with them.

Are you sure the engine is running on all cylinders? When I got my 73, 2.0 one cylinder wasn't firing. It drove but very slowly. I found the valve seats had worn to the point there there was zero lash on some of the valves and compression was very low. Setting the lash to specs got all cylinders above 100 though not equal. Hope one of those helps.