GM Performance 350/290 motor question

[99-LS1-SS]

I have a 1984 K10 longbed with a slightly beefed up 700R4 and a 2400 RPM converter. My axles have the stock 3.08 gears running 33" tires. My truck originally came with a 305 and to say it was weak was putting it mildly (it wouldn't spin the tires on dirt).
I have recently installed the GM Performance 350 motor with 290 horsepower. The carb is a Holley Street Avenger 80670 (670 cfm) with vacuum secondaries. I'm running factory exhaust manifolds through the factory exhaust.

I have a couple of questions relating to the carb and the motor.

My engine question is; in the installation/setup instructions it states that the vacuum advance should NOT be used with this engine. I have copied and pasted directly from their sheet below.

"Set initial spark timing at 10º before top dead center (BTDC) at 650 rpm with the vacuum advance line to the distributor disconnected and plugged. This setting will produce 32º of total advance at wide-open throttle (WOT). The HEI vacuum advance canister should remain disconnected. This engine is designed to operate using only the internal centrifugal advance to achieve the correct timing curve."

What are your opinions on this statement?


My carb question is related to a slight bog right off of idle that is more pronounced when the engine is cold. I've been watching a lot of Holley's videos and most of them state that you need to increase the accelerator pump nozzle size to cure this hesitation but, in the same Holley video it states that if you have a heavy vehicle with highway gearing then you need to lower the accelerator pump nozzle size. If I understand how these carbs work, when the choke is on the engine is in a richer state than when it is warmed up. If that is the case and the stumble/hesitation is worse when it is richer then it seems like I might need to decrease my accelerator pump nozzle size. I believe my carb came with #31 nozzle. If I follow Holley's 3 number jump at a time, I should go down to a #28 nozzle. Does any of this make sense or am I way off. I'm trying to learn more about carburetors and I want to learn the right way.

Do any of you guys run a 350/290 crate motor with a Holley 670? If so, did you have to alter it out of the box?

Thank you for any help.