Holley Street Avenger 570 Idling Problems

pishta said
When any port above the throttle blades (ported) start to pull, Is it the vacuum from the intake that is now being "seen" or is it the fact that high velocity air traveling over a tube end pulls the column from the tube? Thought it was the latter, like the vacuum secondary of a Holley 3310, only opens when secondary air passes over the vacuum put inlet. Either way, I would get the front idle transition slots perfectly square and then adjiust the secondary throttle stops to get desired idle, not the throttle stop.

Pishta, As best I can tell, the answer can be both! Normally, the idle port and transfer slot primarily see the pressure difference from above the throttle to below the throttle. Basicly in plain language, manifold vacuum. Then when running at wide open throttle, the pressure drop created in the venturis is usually so much higher than created down in the throttle bore that the the idle system actually reverses.

However, some of the folks who have done lots of testing (and sometimes even filming) have seen cases where the idle port flows fuel at WOT. This is the latter situation you describe. My guess is that a restricted carb (like many circle track classes) might do this, and other cases it would depend on the details of the carb as well as the engine.

Here's a post by one of these gentlemen describing the typical situation
http://racingfuelsystems.myfunforum.org/viewtopic.php?f=25&t=219

If you want, I can try to dig up posts on when it goes the other way. I'm pretty sure Shrinker wrote about it on the old Innovate forum and maybe even provide a photo. Ryan Brown may also have video of it - in the video of the 4412 2bbl it looks to me like the idle circuit shuts off, but then fuel gets pulled thorugh accelerator pump! - http://www.ryanbrownracing.com/Video.html