Idle air bleeds or transition tubes

[Kahuna]

I've been trying to get the tune better on my carbs:
They are old Holley 4 bbl carbs on my flathead and have run well with good gas mileage and are real responsive. However, the idle speed has always been high (about 1200 rpm), and I haven't been able to get it as low as I want (800 rpm or so), until now.

At the suggestion of a well respected carb guy that works on these exclusively, I blocked the idle air bleeds in each carb. Doing so, allowed the idle to be lowered as much as I wanted. But, this has created another problem: There is a big hesitation off-idle.

My question is- because these air bleeds are not adjustable, how can I determine how much to open them up? I have and can use an AF wide band. Would that be of any value?
I know the size of the air bleed holes, but am hoping I don't have to do so much trial and error testing.

Any thoughts are appreciated

Jim

[pamotorman]

those idle air bleeds are also a siphon break so the fuel does not keep flowing after you shut of the engine. did you check for that ?????

[bigfoot584]

I'd be checking for vacuum leaks or be sure your secondaries aren't
open to far. NO NO to covering air bleeds except to test, you need
them .

Here check this site out for info it may help.

http://racingfuelsystems.myfunforum.org/index.php

[Tuner]

You can use a smaller area restriction in those tubes.

What they do is important at the very beginning of main nozzle flow. The air flow through the tube constantlys draw the fuel off the bottom of the booster which otherwise would build up until it becomes a full size droplet and fall off all in a big glob and make the engine stumble from momentarily being too rich. Also, they create a booster signal and initiate main circuit fuel flow when the total mass airflow through the main venturi would otherwise be too low to do so.

The initial droplets of fuel which form on the bottom edge of the nozzle at the very beginning of main nozzle flow gradually form into ever larger drops which, when they fall off the nozzle, cause a rich gulp of fuel all at once. The tubes, connected to intake vacuum below the carb, constantly draw the fuel off the bottom of the booster and vaporize it through the high velocity orifice (sonic velocity if the intake vacuum is over 14” Hg) and in so doing prevent the large slugs of fuel all at once.

They also provide a source of booster signal which encourages the beginning of main circuit flow. You are having the flat spot because the lack of that signal is resulting in the main starting later.

The location of the little tubes inside the flow area below the booster and at the downstream edge of the booster concentrates the small amount of air flow they provide and causes that idle air which is bypassing the throttle butterflies to flow through the booster.

[groberts101]

all I could possibly add after such insight as Tuners reply is that if you've lowered the idle speed via closure of the main shafts only?.. you may now be too small on your transition slot exposure.

In some app's.. it's possible to nearly close the secondary shafts almost completely so as to gain back some front idle transition slot exposure and this helps to come up on the booster faster. Also works to avoid the stumble and need for larger "overkilled pump shots" at such low throttle angles and transitions. Any holes potentially created during secondary transitioning, rare and minimal as it may be at times, can easily be covered up with slightly more and/or slightly longer duration pump shots.

Pretty easy to try and fully reverse if no gains can be found.

[Tuner]

groberts101, do you understand this is the Holley carb in discussion in this thread?

[groberts101]

groberts101, do you understand this is the Holley carb in discussion in this thread?

oops.. uhhh.. I guess not. he said "old Holley 4 bbl carbs" right up front and I just ran with it. Apparently I missed the flathead part. My apologies.

[Tuner]

No worries.

[Kahuna]

Tuner, Thanks so much for the information.
I'll post my results soon.
Regards
Jim