I have set of used 40's that I rebuilt and I am having a problem with the pump jets dripping in both barrels at idle. This does of course make setting the mixture impossible and made for some large backfiring out of the exhaust on a quick ride arond the block. Can anyone tell me what might cause this?? The other carb works great and both were assembled at the same time and both floats are properly set. Help!!

First guess....Check that the float (gross main jet) ball valves are closing fully.

also

Check your pump pressure (3-5psi max)

Check your float bowl level, could be to high.

My guess is the physical accel pump system. The little roller on the lever arm that rides a little cam between the barrels (down at the base) on the butterfly shaft gets gummed up and crudded causing dribbles and erratic accel pump actuation.
my 2 pesos...

Seems like all IDFs have this problem.

I agree with Scott that you can look at the linkage. Sometimes to vibration of the motor wiggles the linkage, which can be enough to cause little pump actions, which comes out as drips.

Also, the float level has a 'trick'. You have to raise the float until it TOUCHES the ball, but does not depress the ball.

Thanks for the words of advice. I suppose I'll recheck the the float level and needle because I have seen an IDF flood because the float needle was not square on the seat. Fixed it by turing it around and putting the hanger on from the other side of the float lever. These carbs had a 24 hr solvent bath and all pieces and passages were blown out with compressed air so I am pretty certain it's not gunk or trash causing problems. Back to the garage!!

If you put an aftermarket carb kit in them the float valves that come with them are junk, try the old ones.