Holley Carb kicking my tailpipe!

[Jermz01]

A carburetor? I can barely spell the word - let alone admit to growing up in the generation that had to use them daily on their rides...

That being said - I've had a nice little crash course in "How to dial a carb in"...

Problem is... I'm at that last 10% and I just can't seem to get it...

The problem: off idle stumble (keep reading...)

I have a 4160 Holley 4-bbl (600cfm if I recall) with vacuum secondaries that came on my 1969 C10 (small block 350, mild to stock cam). Stock ignition system. When I bought the truck it had a nasty dead spot right off of idle... sitting at the stop light you'd stand on it - and - wait for it... NOTHING!

Swapped the accelerator pump (30cc)... had a nice little back fire when I was screwing around in the garage (stupid me)... since the carb is bit older - no "back fire protection"... so long power valve...

Pulled the primary and metering block... cleaned everything! New 7.5 power valve (the vacuum measured 15 inches of mercury). The valve I pulled out was a 6.5...

Cleaned everything up - put back together with the new 7.5 valve, another new 30cc accel pump, new gaskets... dialed the secondaries to just hang a hair off the slots... idle mixture screws are 1 and 1/2 turns out on both sides per Holley's recommendations...

Truck starts great... idle to WOT is great... idle to a very mild part load is terrible... the engine stumbles, stutters, shakes a little... at stop signs it's all or nothing with the pedal!!

Is my accel pump jacked (again)? This occurs in neutral, while in gear... even if you are cruising - back off the throttle and lightly tip in... I have the accelerator pump arm snug but not tight on the throttle linkage (sorry I don't recall all the correct names)... I pulled the accel pump diaphragm off and checked it (again)... no signs of wear, tear, etc. Spring looks and feels good.

I have a one inch phenolic spacer on the carb as well...
Older single plane Edelbrock (plan on changing it)...
350 turbo trans...
Vacuum attached to PCV off of valve cover, to the distributor and one to a vacuum can.

Open to ideas and suggestions???