Holley Carburetor backfiring GMC K20

[silverchev08@gmail.com]

All,
I bought an 85 GMC K20 earlier this year. Previous owner installed Holley carb and distributor. Truck runs fine at higher rpms however when accelerating from idle or driving under 15 mph the truck will backfire through the carb, sometimes killing the truck completely. Not sure if this is a fuel delivery problem or what. Still use the mechanical fuel pump and the carb has never been tuned as far as I know. 
Thoughts?

[bd]

Verify the firing order and timing.  Remove and inspect the spark plugs for fouling and cracked insulators.  Look up inside the spark plug wire boots for traces of greenish discoloration or an accumulation of black soot.  Remove the distributor cap and check for moisture, cracks, carbon tracking and rotor condition.  With the engine off, look down the throat of the carburetor and open the throttle by hand while you watch for positive squirts of fuel out of the accelerator pump nozzles.  Does the pop occur with the choke applied?

[silverchev08@gmail.com]

I'll perform the above and let you know how it goes. Funny you ask about the choke. The light never goes off. wouldn't even know if there is a carburetor choke attachment.  Not sure of it was ever hooked back up When the last guy replaced the stock carburetor with the new Holly carburetor. It's also got a vacuum hose that comes off the distributor that isn't connected to the carburetor. Not sure if this is an issue.

[bd]

Whether the disconnected vacuum advance is an issue will depend on how the distributor advance is otherwise configured and the base timing.  The constant choke light illumination can be evaluated and corrected referring to the thread, GM electric choke operation for Q-jet carbs.  If the Holley uses an electric choke it can be configured to mimic the OEM design and integrate the dashboard choke light into its control circuit.

[silverchev08@gmail.com]

Vac advance was not connected. Attached it to the carb and wala, runs like a champ now!