Carb backfire under load

[Huskerfan33]

Hello, I have a 76 K20 with the 350. It has fairly new plug wires, plugs, distributor cap and rotor. It has bone stock internals, and I just put a fresh Edelbrock 1406 650 cfm on it. I had it running great and then all of the sudden while driving I noticed a lot of engine noise and backfires through the carb at high rpm/under load most during acceleration. I double checked timing, and it is at 8, vacuum is around 15, checked valve lash and adjusted accordingly, new fuel filter and pcv. I'm stumped on this issue. I assume it's not a carb issue because the carb is all but brand new and was running fine before. Any ideas on what else to check? it idles fine and runs on all 8 still.

 

[Snoots]

I suspect a vacuum leak.

 

[77Dmax]

Lean conditions under load will backfire through the carb. Put two jet sizes up in it and drive it

 

[Ricko1966]

How long did you drive it with the new carbureator,before the problem appeared? Why did you change carbureators to begin with?

 

[Sad Sack]

Edelbrock are notorious for needing calibration.

 

[Huskerfan33]

Update, Fuel pump must have been on its way out. While tinkering it quit pumping so i suspect it was weak causing a lean condition under load