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Discussion Starter · #21 ·
You are more than likely experiencing a "Lean Pop"
As others have stated the 670 Street Avenger is not the best choice of carb, but can be worked around with some tuning.

If your power valve is in three pieces then you have a good place to start right there by correcting that problem.
Put a new power valve in same issue the front jets are 65 the rear jets are 68.
 

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More for Less Racer
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How do I do that? Do I loosen or tighten the bolt on the accelerator pump.
Pump shot is adjusted by either changing the size of the pump nozzle (Shooter, or Squirter) by swapping it for another with larger holes, or changing the pump cam on the linkage. Possibly and frequently both.
The spring around the bolt on the linkage lever is just an override protector spring, not a pump shot adjustment.

Can't find the link but my power valve in the carb came out in pieces
The lowdown on the problem with the 670 Avenger carbs:
 

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I can't figure out what's happening with my sbc 350. It idles smoothly and revs fine in park. Under load it backfires through the intake. It has ported 882 heads that are freshly decked with new valve guides, valves and springs, new cam and lifters and new intake with a Holley 675 carb. I checked for a flat cam lobe, vacuum leaks and made sure the firing order is right. Can't figure it out. All I know is when I adjust the mixture screws the rpm doesn't change until the screws are all the way in but I have them out about 2 ¹/² turns.
You are more than likely experiencing a "Lean Pop"
As others have stated the 670 Street Avenger is not the best choice of carb, but can be worked around with some tuning.

If your power valve is in three pieces then you have a good place to start right there by correcting that problem.
 

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I have to agree with the fact that you have a lean throttle opening problem with carbs with power
valves 1 good backfire can blow the power valve and replace it is the only repair. That often happens when first starated. Also low fuel level in the float bowl can cause it. If you’re capable you can block the power valve and rich up the jets and see if that works if so I would replace the carb with one with out a power valve.
not cheap but good long term repair
 

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I can't figure out what's happening with my sbc 350. It idles smoothly and revs fine in park. Under load it backfires through the intake. It has ported 882 heads that are freshly decked with new valve guides, valves and springs, new cam and lifters and new intake with a Holley 675 carb. I checked for a flat cam lobe, vacuum leaks and made sure the firing order is right. Can't figure it out. All I know is when I adjust the mixture screws the rpm doesn't change until the screws are all the way in but I have them out about 2 ¹/² turns.
You're going very lean. Go to work on the carb or replace it. Quick Fuel Tech are great carbs and tend to run fine right out of the box with the idle screws one turn out.
 

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I have to agree with the fact that you have a lean throttle opening problem with carbs with power
valves 1 good backfire can blow the power valve and replace it is the only repair. That often happens when first starated. Also low fuel level in the float bowl can cause it. If you’re capable you can block the power valve and rich up the jets and see if that works if so I would replace the carb with one with out a power valve.
not cheap but good long term repair
If the PV is blown , the carb goes rich . most Holley's in the last 20+ years have blowout preventers . blowout preventers are cheap & easy to install . Running rich all the time ( bigger jets) on the street is not a good idea . You simply have to be better informed than a carburetor .
 

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Discussion Starter · #34 ·
Well I fixed it. As everyone suggested it was running lean. The Holley street avenger 670 had 65 jets in the primary and 68 jets in the secondary. I ordered a set of 71 jets from Holley and they arrived yesterday. I put the 68 jets in the primary side and the 71 jets in the secondary and also went ahead and put my base timing at 12 degrees initial and 35 degrees total timing at about 3500 rpm. Truck runs great now thanks guys.
 
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