This is from nastyZ28 forum......

There are 3 problems on the Street Avengers that cause the stumbles and hesitations:
1. Lean Jetting. The S/A carbs are jetted very lean on the primary side, and extremely lean on the secondary side. Bump primary jetting up about 3 jet sizes and then make the secondary jets 8 sizes larger than the resulting primaries. To save a few bucks on jets, take the jets out of the secondary side and install them in the primary, and then just buy a pair of jets for the secondary side. This gets the carb jetted about right and will do wonders for throttle response and power.
2. Accel pump lever not touching pump cam. I see this constantly on the S/A carbs, and it's really lousy assembly work at the factory: The accel pump lever that connects between the pump cam and the accel pump arm does not ride on the pump cam at idle. The lever is riding on top of the throttle shaft spring and is not touching the pump cam. Only after you rotate the throttle a bit (with no accel pump action) will the pump cam touch the lever and produce a pump shot. To fix this, remove the lever and bend the lever a little more right where you see the "kick-up" bend in the lever where it's supposed to ride on the cam. By giving the lever more of a bend, the lever will not hit the shaft spring and will ride on the pump cam right where it should. This will produce an instant pump shot off idle, and will solve most of the hesitation/stumble issues associated with these carbs. Keep in mind that you will need to re-adjust the accel pump lash adjustment after you bend the arm.
3. Inadequate secondary idle speed setting. On most S/A carbs, the secondary throttle (idle speed) is fully closed. This causes the primary idle speed setting to get cranked up too far in order to achieve a reasonable idle speed, thus exposing too much of the primary transition slot. This, in turn, will cause an off-idle stumble due to lack of transition fuel enrichment as the throttle is opened. The primary and secondary throttles should be open the same amounts at idle - they should be balanced and see equal airflow. See my Holley Setup Paper on how to do this.

Forget about swapping pump cams and accel pump nozzles - that's not the issue, and it won't solve your problems. Fix the pump lever geometry instead...

Now, as far as fuel pressure, no modern 4-bbl needs or wants more than 5 psi at the bowl and most of them would operate better with 4 3/4 psi. More pressure than that can overpower the needle and seat in the bowl and allow the fuel pump to blow raw fuel into the intake manifold, creating a tuning nightmare for you.

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