Car spits/stumbles at low RPM under acceleration.

Hi guys, its been a while, I have been out enjoying my car since I got it finished this past winter and while doing the "shakedown" period I noticed a tuning issue I am having troubles with. I am in no ways a good carburetor guy, but I'm hoping you can help me out there. I have a 67 coupe with a pretty much stock 289 with some headers and a 4 barrel intake with a little Holley 600 vac secondary. The carb is new, with factory settings. The car does great from about 1200 rpm up, pulls strong, there is no rich gassy smell, no signs of richness (no soot on my bumpers or back end of the car). But when I am pulling out of my drive, or at a dead halt and step on it, the car will spit and nearly die (has died a couple of times), but if I feather the throttle up to 1200 or so and nail it, it will get up and go. I am certain that I probably have an accelerator pump issue (either too much or too little) or a timing issue. What kind of advice can you give me? What do you think the timing should be set at? I have an electronically converted points distributor and a c4 auto with the little 2.79 gears if that info helps. The car idles like a dream at around 600 (so no big cam at all). Another thing, If I load up the car (try to powerbrake) it will usually try to die on me, unless I get creative with the feathering, so I know this has to be a related low-rpm issue.

Ugh, just got a new smartphone and that's the main source of my internet now, I havent gotten used to typing with that thing, anyway. I advanced the timing a little bit and that seemed to help, still has a little hesitation in it. I did tune the idle with a vaccum gauge, I also set the timing with a light some months ago, but I really don't remember what I set it at (my light is broke, need a new one). Still now if im driving through town, low speed 25 or so, slow down to turn and then give it a little extra gas (not nail it, but just give it more than I actually need to get back up to speed) the car will spit and nearly die, I can hear a loud thump in the collector of the header from the backfire, but once i feather the pedal it will get back going. It don't always happen, maybe 1 time out of 10, but thats one time too many.

I know about the pump play, I thought I had adjusted it out. The issue is the spring on the pump arm...it gives and has to compress a bit before it has enough tension to actually push down the pump arm. I may try ordering a new spring (or stretching this one a bit) because I can open the throttle with one hand and push down the pump with the other (getting an instant shot) and there is no bog.

I do like quick fuel, but If i keep the car I am really REALLY eyeballing one of those new holley avenger TBI kits...probably be best off sticking my old autolite 2100 back on it honestly...Good Idea for the washers, I'll give that a try.