Stumble at initial full throttle

I have an issue with my 340 4 speed. The engine has developed a low speed stumble. It seems to run fine at high way speed.

I have replaced the coil and distributor cap. The ignition has been converted to electronic ignition.

I checked one of the spark plugs, it looks to have normal coloration. Wires are new.

If the car sits and idles for sometime, it will start to lope, eventually will load up and then quit. The carb is an Carter AVS 4611 which is correct for the engine. It has been rebuilt and worked fine. This problem just started.

Any ideas?

Thanks

Pa karp (Curt Rees)