Manifold vacuum for distributor.
Thread Starter
Le Mans Master
Joined: Jun 2019
Posts: 5,484
Likes: 2,193
From: Loveland
Manifold vacuum for distributor.
I am using manifold vacuum for my distributor. I am using the manifold vacuum port that comes straight out of the intake manifold behind the carb. This is the same vacuum that works the headlight doors. Is this manifold vacuum ok to use for the distributor or should I use the manifold vacuum at the base of the carb? Lars.
I'm not sure why, but when I moved the distributor vacuum from the manifold port to the port on the carb the RPM dropped from 750 to about 550. I thought both were manifold vacuum.
I'm not sure why, but when I moved the distributor vacuum from the manifold port to the port on the carb the RPM dropped from 750 to about 550. I thought both were manifold vacuum.
Last edited by kodpkd; Aug 3, 2021 at 01:33 PM.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Not sure which port on the 'base' you are using. Some are manifold vacuum and some are timed/ported vacuum. Location on the carb will not distinguish which is which. At idle, manifold vacuum will me at maximum; timed/ported vacuum will be near 0" Hg. Put a vacuum gauge on the ports and find the one you need. For the vacuum advance can, it would be BEST to find a manifold vacuum port at the carb. But, it would be acceptable to use the one plumbed directly into the intake manifold. Closer to the carb is 'better'.
Thread Starter
Le Mans Master
Joined: Jun 2019
Posts: 5,484
Likes: 2,193
From: Loveland
The one I am using on the base of the carb is labeled, in a different thread, as manifold vacuum by Lars. This isn't a 4175 but the same ports. I will check the vacuum with a gauge.
Last edited by kodpkd; Aug 3, 2021 at 02:06 PM.
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
Use any vacuum source that draws vacuum at idle.....if it doesn't, don't use it. You probably plugged it in to ported vacuum, which has nothing at idle and nothing until the throttle plates are cracked.
Jebby
Jebby
Thread Starter
Le Mans Master
Joined: Jun 2019
Posts: 5,484
Likes: 2,193
From: Loveland
Well don't believe everything you read. That port labeled full manifold , isn't manifold on my carb. It didn't pull vacuum at idle, it pulled about 14 inches revved up. I'm getting 12 at idle on my manifold port.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Early 4175 and 4165 carbs had a single ported vacuum nipple on the passenger side of the primary metering block. All other nipples were manifold vacuum. After 1973, in order to make the 4175/4165 as "direct replacement" as possible for the Q-Jet, there was a ported vacuum nipple added to the throttle plate on the forward passenger side intended for use to trigger the EGR and EVAP canister purge valve. Thus, on some 4175-type carbs, the 2 side-by-side vacuum nipples in the forward passenger side of the throttle plate are both manifold vacuum. On other 4175s, intended for emissions applications, one of the two nipples will be ported vacuum, thus providing two ported vacuum nipples on the carb (unlike the Q-Jets, which only have 1 ported vacuum nipple). Both Holley and Rochester changed locations for their ported/manifold vacuum nipples depending on year and carb model number, so you can never assume that any given nipple is manifold or ported - you have to test it with your finger or flip the carb upside-down and look at where the nipple exits into the carb's throttle plate.
Lars
Lars
Last edited by lars; Aug 3, 2021 at 06:03 PM.
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