Ported vacuum for vac advance

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Still looking for redemption

Maybe these will help?
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Degnol said:
So... Jim C...If I have one of those, which I do, I can use the engine-side port for my vacuum advance HEI?
Apologies again, they all look the same to me. I had no understanding of ported vacuum vs manifold vacuum. But it makes sense.
Thanks
Ed
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Ed,
Yes, your pictures show a F carb t-body w/ ported vac fitting. Use the ported fitting for vac advance.
 
FJ40Jim said:
Ed,
Yes, your pictures show a F carb t-body w/ ported vac fitting. Use the ported fitting for vac advance.
Click to expand...
I have a carb built in 1971 that has a vacuum port on the valve cover side but it's above the iron base plate in the aluminum body and to the left of the idle screw. SOR Lists this on page 42 of their catalog as a part # 042-01D 10/69 to 9/71 carb. Is this port ported vacuum? What level of vacuum does a 'ported' port pull compared to manifold vacuum?
 
This description is what cleared it up for me.

rgentry said:
I clipped the following from: [urlhttp://www.bob2000.com/carb.htm[/url]



QUOTE:

Vacuum ports and what they do.

Not all Holleys have the same ports, but you can tell by location and size what each one does. Any port at the very bottom of the carb (in the aluminum baseplate) will pull manifold vacuum, which is strongest at high vacuum situations such as idle and slowing down, and weakest at low vacuum situations such as wide-open-throttle (WOT). Common manifold vacuum connections at the bottom of the carb are PCV (usually a 3/8" diameter tube) and EGR/distributor retard/smog device hookups (usually smaller tubes under the primary fuel bowl). The other type of vacuum connection on a Holley is ported vacuum, which is weak at idle, strong at cruise, and weak at WOT. This one is always (if supplied at all-some competition carbs don't have this port) on the passenger (US) side of the carb, above the idle mixture screw, and is generally used for distributor vacuum advance.
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My vacuum port install came to a screeching halt today because I didn't have a drill chuck small enough for a 1/32" drill bit. I ended up getting a dremel chuck from wal-mart so I'll try again tomorrow. I couldn't find .156"(5/32") brass tubing anywhere. I pulled a tube out of an old Datsun carb I had laying around, it's a bit bigger (.198") than what Ed Cook's tech link suggests (.156") but I think it will work.
 
Coolerman said:
I have a carb built in 1971 that has a vacuum port on the valve cover side but it's above the iron base plate in the aluminum body and to the left of the idle screw. SOR Lists this on page 42 of their catalog as a part # 042-01D 10/69 to 9/71 carb. Is this port ported vacuum? What level of vacuum does a 'ported' port pull compared to manifold vacuum?
Click to expand...

If you follow that port when the carb is disassembled, you will find that it goes into the hollow screw that comes up from the throttle body into the fuel bowl. There is also a hole in the bottom of the primary bore that provides full manifold
vacuum to this screw. Therefore, it is NOT appropriate for a dizzy vacuum source.

You can confirm this with a vacuum guage as well.
 
Well I'll be! I tested that port today while working on the Cruiser. Manifold vacuum it is. Guess I'll be drilling the hole when I locate a late model distributor and ignitor. I currently have the Man-A-Fre Delco Remy distributor. I know... I know... Thanks for the description of the port. Any idea what it was used for?
 
Ok, so I've drilled the hole and installed the vacuum port where it is on the non-USA carb. I'm pretty sure I didn't drill through any passages I wasn't supposed to and the tube is pressed in tight with a bit of jb weld.

Am I in the clear here or is there a chance I won't get the right vacuum from this port? I still have to rebuild the carb and put some bushings in the primary throttle shaft bores,so before I do all this work I want to make sure I installed the port right. I'll try and post some pics if it would help.
 
Get a vacuum gage. I use the portable kind that plugs in with rubber hoses. $25.
If the vacuum at your new port is ZERO at idle, it's ported vacuum. If the vacuum
is high, say 18 at idle, it's manifold vacuum. As the throttle plate cracks open,
the ported vacuum will increase above zero.
 
I haven't finished rebuilding the carb yet, so I was looking for a way to find out if the hole I drilled is in the right place before I continued. I'll definately put a gauge on it when I'm done and see what's happening.

If the hole is anywhere above the throttle plate does that mean it's ported vacuum or does it have to be closer to the plate? Has anyone done this mod and ended up with just manifold vacuum? Is this why everyone says you need a bunch of carb bases and you'll end up scrapping a few?

I guess I just have to get my ass in gear and finish this damn thing so I can test it!

Thanks for all the info so far guys, I wouldn't have even thought about something like this without it. :beer: :beer: :beer:
 
As long as it is above the throttle plate it will have "ported" vacuum and not manifold vacuum. It generates vacuum primarily as a function of the air speed past its location by the Bernouli effect. The closer to the throtle plate the more vacuum it will report at partial throttle because the air speed will be slightly higher. I haven't fooled aroung with it personally, but I doubt that it will make a huge effect if it is off a quarter inch, but if it is below the throttle plate it will have a high vacuum at idle.
 
landcruiserrich said:
If the hole is anywhere above the throttle plate does that mean it's ported vacuum or does it have to be closer to the plate? Has anyone done this mod and ended up with just manifold vacuum?
Click to expand...

I'll repost the picture that may explain it. You can see an obvious hole which is below the butterfly(upside down in the pic) look closely at the 12 o'clock position in the butterfly and you will see just a RCH of the ported vacuum hole.
So, at least on my carb, it does seem to have a tiny amount below the butterfly, but the majority is above.

GL
Ed
Enginerebuild 025.webp
 
The port you show that is right at the throttle plate directly above the idle screw hole is the "transition" or "off idle" fuel port or slot that is a part of the idle fuel circuit. Spray some carb cleaner in there and you will see that it comes out the other hole. The vacuum port should come in at an angle just from looking at the location of the tube.
 
So the other (above) the throttle butterfly is the ported vacuum? OK.
Enginerebuild 031.webp
 
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