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Why would I get a reading of 10hg at idle on this port on my Holley 770 Street Avenger carb, when the diagram shows it's for the distributor advance? If I'm not mistaken, this is ported vacuum, correct? And ported vacuum should be zero at idle, right?
About the only thing which comes to mind (directly anyways) is the idea maybe the throttle blades aren't closed at idle e.g. they're slightly open?
Ok Norm, let's run with this if you'll indulge me. How would the throttle blades being partially open cause a vacuum? Is it because manifold vacuum is being pulled down through the partially open blades?
Ok Norm, let's run with this if you'll indulge me. How would the throttle blades being partially open cause a vacuum? Is it because manifold vacuum is being pulled down through the partially open blades?
Well, after re-reading your post...I think it's more likened to the intake manifold vacuum is being pulled "up" above the throttle blades and entering the ported vacuum source located above the throttle blades.
Well, after re-reading your post...I think it's more likened to the intake manifold vacuum is being pulled "up" above the throttle blades and entering the ported vacuum source located above the throttle blades.
How would I verify this? Look down inside the carb?
How would I verify this? Look down inside the carb?
Honestly, I can't answer your question specifically for a couple reasons. I believe my logic is correct in assuming you're pulling intake manifold vacuum into your ported vacuum source. However, I can't say how far open your throttle blades should be at idle on a Holley 770 Street Avenger 1/64", 1/32", 1/8", 1/4"...I just don't know. You asked a good question, you shouldn't have any vacuum at that timed sparked advance port at idle since it's supposed to be ported vacuum (above the throttle blades). How you're obtaining that vacuum just seems logical it's coming from intake manifold vacuum. I mean I hope it's just an adjustment you need to make and nothing serious like a crack. I'd assume there's a specification setting in the Holley manual which demonstrates the throttle blades position in the running idle position.
If I couldn't find it in a manual, I'd probably do something similar to keeping the vacuum gauge hooked up to that timed sparked advance port and lightly push down on the throttle blades until you arrive at no vacuum? Guess anyways.
Ok Norm, let's run with this if you'll indulge me. How would the throttle blades being partially open cause a vacuum? Is it because manifold vacuum is being pulled down through the partially open blades?
Because that's how "ported" vacuum ports work on a carb. "vacuum" is present underneath the throttle blades, but the throttle blades move!
The "manifold" ports are holes located under the throttle blades. The "ported" is a hole located inside one of the throttle bores - should be the front right on a holley - such that when at idle the hole is above the blade so there's no vacuum at that point. Once throttle is opened a little that hole is now below the blade so it sees manifold vacuum. If idle is messed up and the idle screw is screwed way in then the idle position will have the hole exposed. It only takes a few turns of the screw to be "way" off.
Take off the carb and look at the bottom of the throttle blade. You should see an extra hole in the right side bore - in the base plate, not the carb body - that's the ported vacuum.
Because that's how "ported" vacuum ports work on a carb. "vacuum" is present underneath the throttle blades, but the throttle blades move!
The "manifold" ports are holes located under the throttle blades. The "ported" is a hole located inside one of the throttle bores - should be the front right on a holley - such that when at idle the hole is above the blade so there's no vacuum at that point. Once throttle is opened a little that hole is now below the blade so it sees manifold vacuum. If idle is messed up and the idle screw is screwed way in then the idle position will have the hole exposed. It only takes a few turns of the screw to be "way" off.
Take off the carb and look at the bottom of the throttle blade. You should see an extra hole in the right side bore - in the base plate, not the carb body - that's the ported vacuum.
I get that, but there should be zero vacuum on the port at idle, correct?
I'll assume that 950 RPM is with the transmission in neutral which is too high. I don't know what camshaft the engine has but for stock or very close to stock (think RV type) the idle RPM should be no higher than 750 in neutral. If equipped with an auto trans the idle RPM is best adjusted with the park brake on firmly, the rear drive wheels chocked, and the transmission in drive. The idle RPM should be set to no higher than 650 RPM. Then you can check for vacuum at the timed vacuum port. JMTC.
Last edited by Dynoking; Jan 24, 2024 at 05:34 AM.
