Carb flat spot on acceleration

Okay....I have this 1952 Ford Flathead with all the usual go-fast stuff. I have a Holley 390 cfm that I bought brand new about 8 years ago. Let it sit with gas in it during that time and within the last 12 months, I have finished the truck and am driving it locally. My problem is during engine run-up, either on the road or in the garage, I have a flat spot when the squirters empty and the mains are supposed to take over. If I blip the throttle, I get past that spot and the engine continues to accelerate.
I have cleaned out the main jets and inspected the pump and squirter holes...they seem to be clean. Anybody got any other ideas?
Engine is 239 cu.in. bored out .060", Isky Max 1 cam, Sanderson headers, MSD coil, distributor and 6A, and the carb mentioned. Timing was set by light at 10 degrees BTC.
Thanks for any help!

If you're sure the accelerator pump well is emptying, you could install a 50cc kit.
http://www.holley.com/20-11.asp
I'm wondering if you could reduce the diameter of the squirter nozzles to extend the time duration. Have you tried this? Was the mixture too lean to provide good crisp acceleration?

This may help as it sounds like an acelerator pump problem to me.
http://www.holley.com/data/Products/...199R8108-2.pdf

You can get differtent accel pump cams that will change timing and length of the squirt. Holley 20-12 is a kit

Do you have a 2barrel or a 4 barrel 390 holley????
You will not need a 50cc pump with either

However---if your trying toget the 390 4v race carb----the linkage set ups and calibration on those can take some work

Thanks for all the help! I am going to re-check the points mentioned in the carb by breaking it down further. When I first installed the 4-barrel and cranked the engine for the first time, I don't remember having this flat spot in run-up. So, it probably has to be some crud that I missed when I cleaned out the jets and squirter holes before. I will look at the tach today when I crank it and see when this stall is occuring. Thanks again for all the possibilities....I'll let you know what happens.

Just a quick update... I took the carb apart again...this time I removed the idle screws and cleaned and blew out those passages. Seemed to make a difference! Now I'm back to good again. The transition circuit between idle and full throttle was messed up...a gap between idle and mains was happening. I guess when I first "cleaned up" the carb, I didn't take out the idle screws and blow those circuits out. Dumb! It's usually something simple.
I have another Holley...a 4 barrel off an old Mustang... I took it apart this time also and "practiced" on it just to see what was going on. Now I feel like I can really work on a Holley. May even get some cams and squirters and experiment with my old '52 truck. Thanks for all your help and ideas...really gave me confidence to tear into that carb again with more knowledge!