76' C20 hesitates off idle and surges under load

[CowboyJake]

Hey Y'all!
I've got this awesome C20 (4 bolt 350, Edelbrock 4 barrel carb, Accel HEI, th350 tranny)

I picked her up in Washington State and drove all the way back home to the Orlando area with almost 0 issues the whole drive! My alternator went out and I replaced my drive belt, aside from that I got her an oil change, new oil and fuel filters, and she drove like a dream from coast to coast.

My current issue has presented itself on and off for a few months now as I've troubleshot various things. I'm idling phenomenally, when I get her started she comes to life fairly well, but when I try to give her any more than a very light throttle she doesn't want to cooperate. If I very slowly and lightly open the throttle she can build up some speed, maybe about 25mph, but any more throttle, or hitting the throttle too quickly, always results in one or two things.

1. Total loss of power, it sounds/feels like I'm not firing on all cylinders despite everything sounding operational at idle, the engine revs and I go at a snails pace

2. Surging heavily while giving it more than light throttle, jerkin me back and forth in the seat and ultimately not being able to build up enough speed to comfortably drive on any road

Tlr:
I'm surging under load, I'm idling perfectly, no overheating, I'm getting a clean start if the battery is charged, at rest I seem to be getting fine spark on all cylinders,

My best guesses are this:
1.Vacuum leakage somewhere, potentially at carb/intake
2.Bad ignition module/vacuum advance? (Accel HEI)
3. Accelerator pump/carb issue


A couple extra notes in case they're helpful,
1. My first time setting my initial timing I did witness the carb backfire and shoot flames, I suspect this may have shot the accelerator pump and potentially the mounting gaskets

2. I'm aware of bad exhaust manifold gaskets, at least on one side

3. When tuning the carb, the fuel screw and idle speed screw seem to operate normally, but adjusting the air mix screw seems to literally do nothing

4. I do have the Dual tanks option, and I believe there is a small mechanical pump way in the back near the rear passenger Wheel, although I'm not super well versed on this

Below is a list of all the stuff I've done in hopes it would fix it

-Carb rebuild
-Cap/rotor/plugs/wires
-Mechanical fuel pump/new line and hose to carb
-New vacuum lines from carb to transmission modulator line
-New Detent/downshift cable/armature
-new fuel filters/fuel pump test/inspection of all fuel lines for leaks or pinches


Thank you guys so much for your help, it'd mean the world to me if somebody gave me some tips about where to start diagnosing this.

 

[legopnuematic]

Inspect the wires in the distributor to the magnetic pickup, the pickup rotates with the vacuum advance movement. Over the years and many thousand cycles the wires can fatigue and begin internally creating an open condition.

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Try unhooking and plugging the vacuum advance at the carb and see if problems persist/problem changes.

Also while you are in there, check over the mechanical advance, make sure it advances with rpm and returns to the same position at idle.

Also make sure the pins in the cap for the coil don’t get pushed up as you reconnect the 3 pin harness and power feed to the distributor. Have had that happen before, or the terminal gets unseated from the plug and doesn’t make a solid connection.

 

[Ricko1966]

The engine revs but you go at a snails pace. Are you sure you don't have a transmission issue?