Vacuum to Distributor

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May 28, 2009 | 05:36 AM

daddy172005

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From: Greenwood, SC

Vacuum to Distributor

How much vacuum should be coming from the line from the base of the carb to the vacuum advance on the distributor? Should I be able to feel it with my finger? The reason im asking is I can put my finger over it and can't feel anything. Checking the timing gives me the same reading whether it is plugged or not. If their is somthing wrong then what would cause it to not have vacuum? What does it hurt not having vacuum to the advance? ?Thanks in advance.

May 28, 2009 | 06:54 AM

gfw1985

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From: Raphine, Virginia

It works on ported vacuum. At idle (800)rpm there will be no vacuum. As rpm increases, vacuum increases advancing timing. It is designed to increase advance under load. As far as neccesary, I have one old 289HP that runs fantastic without it. Some came with 2 vacuum ports on dizzy, one that retarded timing during startup.

May 28, 2009 | 11:13 AM

fmc400

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From: Austin, TX

gfw1985 is correct; at idle ported vacuum should be zero. After the throttle plates open, ported vacuum should reach manifold vacuum. At WOT, both manifold vacuum and ported vacuum approach zero.

May 28, 2009 | 01:38 PM

montana_highboy

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From: Big Sky Country

Ditto the previous two replies, if you felt vacuum @idle from the v/a hose than you're pulling manifold vacuum and therefore pulling in all your vacuum advance at idle defeating its very purpose.

May 28, 2009 | 09:11 PM

shadowviper12345

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From: North Carolina

Quote:

Originally Posted by daddy172005

if you have a timing light, hook up the vacuum advance and the timing light then have someone step on the gas. as the rpms go up you will be able to see the timing change. if you can see the change then your vacuum is strong and the vacuum advance works. also check for broken or spit vacuum lines, i found 3 or 4 when i bought my truck.

May 28, 2009 | 10:47 PM

daddy172005

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From: Greenwood, SC

Thanks for replying. I gave it some gas and it had vacuum. Whats the purpose of plugging it to check timing if there is no vacuum at idle? I get the same reading either way.

May 28, 2009 | 10:52 PM

fmc400

MSEE

Joined: Apr 2004

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From: Austin, TX

Not all vacuum advances source from ported vacuum; some source from manifold vacuum, in which case there is vacuum at idle. Plugging off the vacuum advance ensures that you're at base timing.