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  1. #1
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    Default Do i need vacuum on HEI dizzy?

    Hi all,
    I have a 350 with a new Mallory HEI dizzy and just setting the timing.
    With the vacuum removed and plugged i have set total timing at 36deg at 3000rpm (mechanical stops around 2200rpm from memory) and this leaves me with 18deg at idle 850rpm
    When i connect the vacuum (carbi base vac connection) i get around 40deg at idle, is this normal?

    I havnt driven it yet but seems pretty good as is without vacuum so do i need it?
    And if so is the carbi metering plate vac port better to connect to?

    Im no expert on timing engines just trying to decipher what i read online.

    Any advice?
    Last edited by 78HZSBC; 06-01-2021 at 10:13 PM.

  2. #2
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    Default

    Anyway ive decided i do need vacuum and will leave it connected.

    Running it again today these are my figures if anyone is interested.

    No vac
    idle 18deg 800RPM
    36deg at 3000RPM

    Vac connected to full vacuum at the carbi base
    idle 30deg 800RPM
    60deg at 3000RPM

    Before i take it for a test drive do these figures sound normal?

  3. #3
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    You have a couple of things going on.

    [1] The t/blades are open too far at idle, exposing the ported vac adv port to manifold vacuum. Hence the increase in idle timing. Most aftermarket VA units add about 30*. You are getting only 22*, which might mean the VA is non-adjustable & engine vacuum is low OR you have an adj VA unit which needs adjusting [ softer spring setting ].The way it is currently is likely to cause erratic/fluctuating timing. The VA often gets blamed for this [ no good, doesn't work... ] when the culprit is the installer.

    [2] Does the engine have larger than stock cam, which would explain the t/blade position? The fix for this is to drill the t/blades for bypass air, which will allow you to close the t/blades so you have 0.040" or less of transfer slot showing below the blades at idle.

    [3] Does it idle smoother & at a higher rpm with VA connected? I'll bet it does! If yes, that tells you the engine LIKEs & WANTS the extra timing at idle.

    [4] Do you need VA? Yes, if your dist has an adv curve. And it should be connected to manifold vac, NOT the useless ported VA. But don't take my word for it. From David Vizard, author of over 30 auto books & 3000+ magazine articles. This was in PHR magazine Nov 04: "At idle & low speed operation, the amount of adv reqd to most effectively utilize the air & fuel entering the engine can be as much as 50-55 degrees. This is handled by the VA: a function many hot rodders believe is not needed because their favorite drag racer does not use it. Now is the time to listen up & listen good. A functional VA is the single most effective camshaft tamer you can get. By taking the time to hook up the VA to a MANIFOLD vac source you can get a big cam to idle as it were about 20 deg less than it really is..."

    [5] More on MVA by a retired GM engineer. Scroll down to post #6: www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html

  4. #4
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    Default

    You have a couple of things going on.

    [1] The t/blades are open too far at idle, exposing the ported vac adv port to manifold vacuum. Hence the increase in idle timing. Most aftermarket VA units add about 30*. You are getting only 22*, which might mean the VA is non-adjustable & engine vacuum is low OR you have an adj VA unit which needs adjusting [ softer spring setting ].The way it is currently is likely to cause erratic/fluctuating timing. The VA often gets blamed for this [ no good, doesn't work... ] when the culprit is the installer.

    [2] Does the engine have larger than stock cam, which would explain the t/blade position? The fix for this is to drill the t/blades for bypass air, which will allow you to close the t/blades so you have 0.040" or less of transfer slot showing below the blades at idle.

    [3] Does it idle smoother & at a higher rpm with VA connected? I'll bet it does! If yes, that tells you the engine LIKEs & WANTS the extra timing at idle.

    [4] Do you need VA? Yes, if your dist has an adv curve. And it should be connected to manifold vac, NOT the useless ported VA. But don't take my word for it. From David Vizard, author of over 30 auto books & 3000+ magazine articles. This was in PHR magazine Nov 04: "At idle & low speed operation, the amount of adv reqd to most effectively utilize the air & fuel entering the engine can be as much as 50-55 degrees. This is handled by the VA: a function many hot rodders believe is not needed because their favorite drag racer does not use it. Now is the time to listen up & listen good. A functional VA is the single most effective camshaft tamer you can get. By taking the time to hook up the VA to a MANIFOLD vac source you can get a big cam to idle as it were about 20 deg less than it really is..."

    [5] More on MVA by a retired GM engineer. Scroll down to post #6: www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html

  5. #5
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    Default

    To answer your question my cam is 230/236 hydraulic.
    Not sure if it idles smoother with vac connected.
    I believe my vac advance is adjustable as i can put a 3/32 allen key in the vac hole and can feel it turning something but havnt adjusted that from out of the box.

    What kind of numbers should i be looking for at idle and at 3000RPM with the vac connected?
    Im assuming i can adjust this.

    Its not that im trying to get peak performance right now just want to have the timing at an acceptable area so i can drive it knowing the timing is ok

  6. #6
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    OK.

    Disconnect the vac adv. Warm up the engine & put into gear if auto trans, engine idling. Loosen dist & turn it slowly CCW [ advances timing ]. Listen & feel. Keep advancing dist until you get highest rpm [ will be smoothest idle, highest vacuum ]; toggle dist to make sure you have highest rpm. Now check what the timing is. Say it is 34*. That is the 'sweet spot' for that combination of parts. If you run 18 init, then the VA needs to add 16*. Most adjustable VA units have a total of 30-32*. So you need to make a stop to limit it to 16*. Crane units come with a notched plate [ can be bought separately ] that makes this easy, but quite easy to make a limiter out of 16g steel that is secured under the mounting screw. So that is the amount of VA.

    The AK adjusts the rate. Turn it fully CW. This is the softest spring setting/lowest vacuum. Engine idling, in gear, turn AK CCW 2 turns & re-check timing. Keep going 2 turns CCW & checking timing until timing changes or becomes unsteady. Go back CW one turn at a time until timing is steady again. Then go 1-2 turns CW. Done!!

  7. #7
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    Default

    Edit
    Last edited by 78HZSBC; 07-01-2021 at 11:21 PM.

  8. #8
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    Yes good info on the above link Geoff so thanks for that.

    Ok so ive come to the conclusion i need to reduce my vac advance a little, i will try the "sweet spot" timing but until then ill calculate total timing being 36deg.
    So if i have 18deg idle and 18deg mechanical i need vac advance to be around 14-15deg to give me a combined timing of 50-51deg at all in 3000RPM.
    Hopefully that will reduce my idle deg to around 21deg with vac connected.

    Reading up on my 85 series Mallory HEI i adjust this with the allen key so will try this when i have a chance.

    Anyway yeah my head hurts now with all this lol so hopefully works out in practice
    Last edited by 78HZSBC; 07-01-2021 at 11:23 PM.

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