I have a new Holley 500 on a new 390. I went through the float adjustment and the idle screw adjustment. It idles smooth and has 19" vac. When you throttle it, it gasps. If you pull the choke half way closed, it throttles like as dream. No gasping when thrown to WOT. That tells me it has an air issue? I did a vac leak check with propane and can't find anything of note. Funny thing is, this gasping issue was not an issue when I stopped running the rig 2 months ago when the alternator went out. It throttled fine. Any ideas?

 

I went back out and plugged away at the issue while I still had a few more minutes of daylight and found that I have no fuel coming from the nozzles for the accelerator pump. I pulled the front bowl and inspected everything and it does pump fuel. Just isn't getting up to the nozzles? Checked the acc pump adjustment and it is good. Not sure where to go from here Why did it work before? I have clean fuel going to the carb. There are actually 3 inline filters before the one on the carb, All are clean. I loosened the screw that holds the jets in place and pumped the throlttle to see if there was fuel coming out but had plugged jets and I get no fuel. Looks like I need to pull the carb and verify all the ports are clear.

 

It sounds like you're on the right track with potentially clogged squirters.... Also, remember that Holley has a tech line available.

 

I've just finished installing a Holley 4412S (500 CFM) on my 390 as well. From what you describe, I would also be looking at the accelerator pump circuit, which it sounds like you are already onto. Once I got that set right, the momentary lag/bog down is gone.

Another thing is that there is a kit of different cams that you can use to fine tune the way the accelerator pump operates once you get it squirting again. Some ease the pump in while others push the pump hard right away. I bought the set of cams and can see that there is a huge difference from one to another, but have not yet got to the point of testing them all out. That's the problem with Holley's, they seem to require a lot of time consuming trial and error to get things just right.

Just out of curiosity since I'm also still fine tuning things:
Do you have the 50cc diaphragm or the 30cc diaphragm for the accelerator pump? Did you modify the power valve? What size nozzles are you using (I'm thinking of going with the 68 or 66, seems a bit rich with what's in there now).