is my distributor vaccum port timed or full

[pete29]

I have a 72 chevy 3/4 ton with 350 and hei distributor from summit. I'm tryin to put a edelbrock carb on it but not sure if the vaccum port is timed or full. Anybody Know?

[68gmsee]

I assume what you are referring to is "ported" versus "full vacuum." If that's the case, then look at the carburetor where the butterfly shafts is. Normally below that area or base of the carburetor is full vacuum and above it is ported.

Not sure about Edlelbrocks but that's the way quadrajets, Rochesters are.

[Indyuke]

Most likely ported. They stopped using full vacuum in the fifties and sixties, I think. Someone else will chime in to say for sure.

Oops! Didn't read the original post fully. If you have it on the truck, you could always put a vac gauge on it while the truck is running.

[Rollie396]

Looking at the front of the carb,the left hand port (above the throttle plate) is ported and the one you want.

[jk396]

i was wondering how to get a complete engine vacuum routing for a 72 402 and a 72 350 both automatics,the assembly manual i have does not show any specifics

[LONGHAIR]

You really need to check the specs of the distributor....and how it was intended to be run. Stock specs of the truck and/or carburetor are totally meaningless.
"Ported/timed" vacuum is a crude emissions gimmick from the late '70 and early '80s. The distributors were calibrated differently to be used that way. The points type distributors from before that, and most of the aftermarket units are made to be run with full vacuum. Why the heck those carburetor companies think that they can tell you how to run your vacuum advance is totally beyond me. They don't know what you have, and it makes absolutely no difference to the carburetor at all.

I would stay away from ported vacuum and any distributor that is intended to run that way. It is not good for anything and is actually leaving both performance and fuel economy on the table. While you may or may not notice it on the water temp gauge, it will definately make your engine run hotter in the exhaust phase. Manifolds, head pipes, and underhood temperatures will go up quite a bit. This is/was an intended effect that burns up some of the un-burned hydrocarbons in the exhaust. It is essentially "late" timing at low engine RPM because the vacuum advance is not activated.
However, there is more centrifugal advance built into these distributors to "compensate" for this at higher RPM, so you can't just switch it to a full vacuum source.....or you end up with too much total timing.

[spinning wheels]

Longhair, so would a stock GM HEI from the late 70s early 80s be a ported vacuum dist.?
Just curious, as I was going to drop an HEI from a '75/76 GMC 1 ton, into my 3/4 ton '72 truck. Currently I am running a points dist. with Pertronix module and a MSD blaster 2 coil. My carb is a Edelbrock 1406, with a stock turbo 350.
Any advice would be appreciated.

[jk396]

thanks so much for the info.