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Discussion Starter · #1 ·
Hey all. What does it mean if you are pulling 12 in vac on the ported side at 550 rpm? Would it be the carb? I read it somewhere but i can't recall what was stated. Thank's for reading my post and any help provided..
Keith
 

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Hey,
Ported vacuum is attained by opening the throttle a bit, manifold vacuum is present with the throttle plate closed. Having said that for normal engines with stock camshafts 12" is low. The list contains things like a large vacuum leak, real retarded timing, or a engine with real low compression. In most cases the engine would be running rough. :sweat:
 

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Discussion Starter · #3 ·
vaccum

driveability said:
Hey,
Ported vacuum is attained by opening the throttle a bit, manifold vacuum is present with the throttle plate closed. Having said that for normal engines with stock camshafts 12" is low. The list contains things like a large vacuum leak, real retarded timing, or a engine with real low compression. In most cases the engine would be running rough. :sweat:
Thanks for responding to my post. I Guess I should have been more concise. I understand how vacuum works in relation to full manifold vs ported vac. Manifold- full at idle. ported-none at idle. I am pulling 19 in rock steady on full manifold which indicates no vac leak. The problem is I am also pulling vac on the ported side (at low idle) as well which should be zero. If the problem is a vac leak wouldn't it also show itself as a vac leak on the manifold side? As I stated in previous post I read somewhere what the problem is but I forget although I believe they said the problem lies in the carb. Thanks again.
Keith
 

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If you are pulling vaccum at idle on the ported outlet of your carb it simply means that the butterflies are open enough for vaccum to show up above the butterflies. If you need to reduce the vaccum you need to close the butterflies and adjust the idle with the idle mixture screws.

That being said I am not sure why you would need to reduce the vaccum signal is this causing a problem for you?

Ric
 

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Discussion Starter · #5 ·
vaccum

Ratchet said:
If you are pulling vaccum at idle on the ported outlet of your carb it simply means that the butterflies are open enough for vaccum to show up above the butterflies. If you need to reduce the vaccum you need to close the butterflies and adjust the idle with the idle mixture screws.

That being said I am not sure why you would need to reduce the vaccum signal is this causing a problem for you?

Ric

Hey Ric. Well yes, it will cause a problem. If one is set up for ported (no vac at idle) and is pulling a vaccum on said ported side you will bring vac ad in at idle instead of off idle, thus reversing the way ported is supposed to operate. BTW, butterflies are closed. Thanks. Have a good one!

Keith
 

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budynabuick said:
Hey Ric. Well yes, it will cause a problem. If one is set up for ported (no vac at idle) and is pulling a vaccum on said ported side you will bring vac ad in at idle instead of off idle, thus reversing the way ported is supposed to operate. BTW, butterflies are closed. Thanks. Have a good one!

Keith


If you have 12"hg vacuum at your ported vacuum connection your throttle blades are above the port. If they are fully closed and still above the port, you have a problem with the blade installation.
Having vacuum at the port all the time doesn't reverse the operation, it just provides constant vacuum.
 

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Have you altered the engine? cam, compression, heads, etc? How did you alter it? Tell us everything you can about the engine.

Sounds like (if you've altered it) you may have to bring more initial timing in and then limit mechanical advance to maintain the same total. That will let you drop the idle screw down. If you have a really lumpy cam you can drill a .018" hole in the throttle blades to supply enough air for idle while letting the blades close enough to cover the ported hole.
 
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