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Secondary main well air bleed tubes

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novadude:
Let's talk about secondary air bleeds.  I've noticed that most Chevy carbs are 0.026-0.028".  Cliff's book recommends drilling all to 0.036", and I believe many BOP carbs came with .033-.036" tubes.

What is the effect of increasing the air bleed size?  I am thinking that this might make the secondaries richer on initial opening - bigger bubbles lead to better initial flow when fuel starts moving thru the nozzle.  I am also thinking that larger bleeds may trend a little leaner at higher flow due to more air mixed with the fuel.  Basically, the slope of fuel curve relative to air flow would change.  Am I correct in my thinking?

How much impact does the secondary air bleed have?  Has anyone done back-to-back tests changing nothing but the air bleed size?

77cruiser:
Somebody did do a test on this but I can't remember where. Might have been Class Racer, but just not sure. I don't think it was here.

novadude:

--- Quote from: 77cruiser on September 18, 2025, 10:49:47 AM ---Somebody did do a test on this but I can't remember where. Might have been Class Racer, but just not sure. I don't think it was here.

--- End quote ---

If you remember where, please share.  Do you recall the conclusions?

77cruiser:
If I remember right a couple HP up to .043. Might have been on the other carb forum that kinda died out. Seems like I asked Tuner & I got salt to taste, so I take it whatever works.

70GS455:
You are correct in your thinking.

The 0.036" did not work for me. I had a post earlier where it kept going lean at high rpm. One thing to note is that I'm near 600 HP with my setup so in order to get there the air door has to be open as far as possible (1.240"), yet I'm still 2" of manifold vac at WOT at the top of the pull. So there is less sec venturi effect vac to pull fuel.

For my 7043240, I reduced them to 0.025" and got really close with CE rods before the engine swallowed a screw. Next I will try 0.022" (the Ebrock RPM comes with 0.020"). I also closed the lower end of the tube and added extra emulsion holes like Tuner suggests although I don't agree with all his recommendations..

There is really good info here:  https://www.tapatalk.com/groups/racingfuelsystems/viewtopic.php?p=16572#p16572

See the pic in the link above

So I don't think the 0.036" will work with higher HP applications. Unless you can close the air door a lot and run really small tip sec rods ( which I don't have) .

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