Idle Air Bleeds - How well do they work?

[Big Red Mach 1]

With interchangeable air bleeds on a Quick Fuel or Holley type of carb, how big of an impact do the idle air bleeds make per size? A certain number of us have had to drill out our butterflies in order to get enough air at idle so that we can keep the idle screw turned way down so that we don't over expose the transfer slot. Would a significant change in air bleeds have done the same thing? Is there any rule of thumb on how many air bleed sizes to go up when trying to better your air/fuel mixture? I'm playing with a new to me 750 QF carb looking to possibly buy it, and the current idle bleeds are 71's and I can't idle at all without cranking on the idle screw. I don't want to drill the butterflies on this one especially before deciding to buy it, but don't mind spending a few bucks on a set of bleeds if they will get it done.

 

[Cleveland Crush]

Lots of variables Big Red. Your idle air bleeds not only impact idle mixture but also the idle transition to primaries and cruising speed mixture. But to your question, about 10% +/- from the factory spec bleeds would really be pushing it. Swapping to slightly bigger air bleeds  may help you. Holley wet flows their carbs and air/fuel jets them accordingly at the factory. Try two air jet sizes to start the process and keep notes of your changes and results and proceed from there. Also consider getting Holley/QF carb tuning book and read up on the process. Good luck with it.

 

[jpaz]

You can get some more idle from the secondary side of the carb too. I opened the secondary butterflies slightly which helped me keep the primary transfer slot from being opened to much. I did this on my QF 735.

 

[basstrix]

In answer to the question, of whether or not a larger IAB would have done the same as drilling the butterflies, the answer is no. The Idle Air Bleed, in conjunction with the Idle Feed Restrictor control the mixture of air and fuel in the low speed / idle circuit. This wouldn't have much change on the opening of the butterflies which was exposing too much (presumably) of the transfer slot. More likely, it (larger IAB's) would require slightly more opening on the idle mixture screws...but not a whole lot.

Some carbs now come with an air screw in the center beneath the air filter stud. Some very old Holleys came with that screw, but I think it was Barry Grant that sold a retrofit kit back in the 80's-90's.

Larger IAB's will act to lean the entire idle circuit, while smaller will richen it. IFR's work opposite of IAB's. In general, the effect of air bleeds tends to increase with RPM.

I've attached a comparison between .072 and .077 IAB's on a holley 3310 that I've modified for replaceable air bleeds. The comparison is done at 2000 RPM where the engine should be running only on the idle circuit. I think the boosters come in around 2400. This produced a 0.8:1 change in AFR on average.

I wouldn't recommend going down this tuning path unless you have an AFR gauge and plan on trying several different changes. I also attached a screencap of the spreadsheet I use to take "notes". It can get rather involved, to say the least lol

 

[Big Red Mach 1]

Thanks guys! Especially for all of that data basstrix. I'm going to take the advice here and not change them. I'm going to pass on buying this carb and stick with my QF735 that is pretty dialed in right now. I don't want to do a lot of work on another carb that might only get me pretty close to where I am right now.

 

[Stanglover]

Thanks guys! Especially for all of that data basstrix. I'm going to take the advice here and not change them. I'm going to pass on buying this carb and stick with my QF735 that is pretty dialed in right now. I don't want to do a lot of work on another carb that might only get me pretty close to where I am right now.

A QF 735 was what I was considering swapping my 670 for, until QF, now Holley, dropped it. I'd keep it for sure.

 

[jpaz]

I have mine dialed in pretty good. If you want some more details, let me know.