Backfires through carb when stepped on

[Big SW]

One night my pickup quit me (bad ignition module), so I fixed it and drove home from my buddy's place. Next day I fired it up to put it in the shed and it backfires through the carb when stepped on. The old carb was a leaky 600cfm Holley. Since leaking and was monkey rigged (from previous owner) I bought a 1905 650CFM AVS2 Edelbrock. It still backfires through carb. I checked all vacuum lines the entire valve train. It has new coil, rotor, cap, plugs, and wires. The valvetrain is also in proper working order. I also moved around the timing a bunch between 4 degrees to 12 and it still backfires. Also, the wires are in the correct firring order 1-8-4-3-5-6-7-2. Need help fast it's my daily.

 

[CalSgt]

flat cam maybe... Any noticeable valvetrain noise, ticking etc.?

 

[Big SW]

no sounds from any valvetrain parts and they all are getting oil. if it was a flat cam wouldn't it do it all the time not when I step on it. if you bring it up slow like granny style it works perfect.

 

[squaredeal91]

18436572. Should be cast into intake manifold. And very well could be flat cam. Had the same scenario with my first truck, had a 350 with several worn cam lobes. Amazing it ran as good as it did. I would Check your valve lift.

 

[squaredeal91]

Or maybe Could just be as simple as needing a heavier accelerator pump squirt?

 

[Trucksareforwork]

1-8-4-3-5-6-7-2

Just want to make sure you know this is wrong: Correct is 1-8-4-3-6-5-7-2 (note the flip of 5-6 to 6-5).

 

[Big SW]

My bad on the firing order. I also just put a new timing gears and chain in it about 500 miles ago, if that helps with anything.