where to get manifold vacuum

[Randall K.]

Hello Carb Experts...

I have a 1972 454 w/ auto trans. The correct carb is a 7042216 - I acquired a 7043216 shortly after I bought the car, as it had a generic GM truck carb on it at the time.

I'm in the middle of mapping my advance curve, and was wondering where the best place was to pick up manifold vacuum on this particular carb. Or, do I already have it???

In the attached photo, there is a capped port on the base of the carb - directly below where the vacuum hose attaches to the choke pull off. Would this be a good location? Right now, the vacuum line for the advance is obtained from the port in the second picture, right below the mount for the idle solenoid.

Thanks
Randy
carbFrontAtChokePulloff.jpgcurrentVacuumAdvanceSource.jpg

[Richard M.]

Re: where to get manifold vacuum

Randy, Typically, if the port is above the throttle plates, it's "ported", and if below it's "manifold".

But the best way to verify which is which, is to connect a vacuum gauge to each. If it's reading max at idle it's manifold. If reading minimal/nothing at idle, then when you throttle up slightly above idle and get a reading, it's ported.

Another way without the vac gauge, is simply put your fingertip over the ports and do the same sequence of tests. Your finger will tell you.

Rich

[Edward J.]

Re: where to get manifold vacuum

To add to Rich's post another source is to use a T and tap into the choke pull-off.