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

 

[Broton2]

I’m having a similar issue with my 76 K20, it’s got a new distributor, plugs, wires, and fuel pump. It starts to backfire out the top of the carb as soon as it’s under load. It idles and runs great but as soon as the secondaries go to kick in it pops like you wouldn’t believe. It’s got the OG quadrajet that’s been rebuilt. I just timed it and have been stumped on it for a couple days now.

 

[ChuckN]

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.

If it is a 1406, it is a 600 cam, but more importantly it is jetted conservatively and also, the accelerator pump nozzle is amongst the smallest Eddy uses. As @Ricko1966 stated though, if it was running fine and suddenly changed, then it could be something different.

If it was fine before and now is suddenly not, look for vacuum leaks. If is brand new install, and is backfiring, it’s easy enough to change the accelerator pump arm to the most aggressive setting (the most rearward slot).