Last week I posted about a dozen or so pages from an old Ford High-Performance Engines guide. This section did a fairly good job explaining the workings of everything from the fuel pump to the intake manifold without getting too deeply technical about it. https://www.jalopyjournal.com/forum/threads/fuel-system-refresher-course.1271372/#post-14574090 Well while scanning and posting this information I gave it bit of it a re-read as a sort of refresher course. It mostly seemed to be familiar enough to me until I got to a section describing the operation of a Holley vacuum secondary 4 barrel carb. In the picture below it shows a schematic and points out something called a "Killer Bleed". In the original post I mentioned that this term was something new to me and asked if anyone knew its function or purpose. There was no response and after a few days of course the thread dropped further back in the page ranking. So I tried to do a little online research on Killer Bleeds but have still not had much luck. The little info I found involved idle air bleeds and emulsion tubes and anti-siphon ports that all involved fuel mixture. But this Killer Bleed, at least the way it's illustrated, seems to be simply a vacuum port tied into the secondary air valve diaphragm. So does anyone know what this vacuum port actually does or why it's there? Or is this one of those situations where I need to call in @carbking for a consultation?
Most bleeds are used to moderate pressure. My best guess is that it somehow works with the check ball to protect the secondary diaphragm. Many times I have removed the check ball to make the secondaries open faster and I know that works. One time I tried plugging the "killer" bleed but I couldn't feel any change so I never messed with it again.
The vacuum at that point in the Venturi’s can be fairly large. I think that term “ killer” was used to lessen the vacuum there, effectively “ killing” the vacuum. I think Holley used that bleed to fine tune when the second aries opened. Just my opinion with nothing to back it up! Bones
Looks to me to just be the vacuum source for opening the secondaries. As Bones point out, that's a high-vacuum area under acceleration, looks like the logical place to put it...
Thanks for checking in on this matter. So far this is more appropriate information than anything else I've found. I think I see what you're saying but where does the check ball go in that vacuum circuit? I sure don't see it in that illustration and I don't have that much of a background tinkering with Holley carbs. That kind of makes sense but I always thought of the various diaphragm springs as being the only means to control the rate of opening. Also the two vacuum ports look to be located in two different diameters of the primary throttle bore. I'd expect that the upper and lower ports would each supply a slightly different vacuum signal. Though I have no idea why you'd want that "mixed" vacuum signal.
Look at the location of the primary pick up….it is a little below the smallest point in the Venturi. The “ killer” is a little above that point. Makes all the difference in the world as to wether you have vacuum or pressure! In the Fire service we can inject foam into a fire hose that has 125 plus psi, using this principle! Bones
Looks like a pressure compensator or an air bleed to equalize pressure or a breather per se for the secondary diaphram. JMO. No reason to do that other than to bleed off vacuum. Or equalize it. You can bet your **** though they figured it out on a dyno. LOL Lippy
Claymart, that little ball is really not a check valve! If you look closely you will see a “ cut” in the seat where that ball sets. It is used to restrict the vacuum to open up the rear two barrels somewhat slowly! Some folks take it out , figuring it will open them earlier. The rear two barrels have to have vacuum from the venturies , bled, goes through t this restriction and then has to battle the force of the spring! It’s a wonder they ever open at all! Bones
The picture is misleading. Diaphragm air feed is in the primary venturi for initial activation but the killer one goes to the secondary venturi adding opening rate or slowing it depending on what the engine will take.
Air speed through the throat creates a low pressure area above the venturi. The higher the speed, the less the air wants to bend. At low air speeds the air can conform to the shape of the area above the venturi, and has higher pressure. This "kills" the low pressure signal in the venturi.
Well now I think we're gettin' somewhere! This Hot Rod/Motortrend tech article from a few years ago had some interesting photos and helpful tuning tips. https://www.motortrend.com/how-to/how-to-custom-tune-holley-vacuum-secondary-carb/ Finally found a photo and illustration showing a check ball in place in the diaphragm housing. So that may or may not be one anomaly in Ford's illustration. Or Ford decided that this particular OEM model Holley needed to use a "Killer Bleed" instead of a check ball. I found this to be even more interesting. The illustrations that showed the check ball also showed only one vacuum port in the primary throttle bore and it shares a p***age with another single port in the secondary throttle bore. Which could also mean an error in Ford's illustration. Or it could just as well mean that Ford decided that by just adding a Killer Port on the primary side and not using a check ball cured some issue they were having with a particular carb/engine combination. Flip a coin. You call it. Six of one, half a dozen, I'm thinking. Either way, I'm pretty well convinced that you'll probably never see this second Killer Bleed in the primary throttle bore except in some 70s or earlier Holley carbs and/or possibly Ford specific applications. Though I still don't know exactly how it was supposed to work.
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