I agree with Dr Charles. Most likely lean in the transition port. Could be too much timing or such, but if its fuel, then the initial throttle position may be out of the working range.

A couple of ways to verify. In neutral, slowly increase throttle from idle. If it hesitates or misses, then its idle circuit. If not, then try it with load on it - that is driving it opening the throttle as slow as you can. If no hesitation, then look at pump shot.

If it misses in the tests, try repeating with richer idle mix and see if that helps.

On the Holley type designs, its very important the primary side throttle position at idle reveals .020 to .040" of transition slot below the blades. Then readust the idle mix using the screws.