[Formulajones]

Originally Posted by Kenth

Front lower passenger side in throttle plate is the EGR port, this port is too weak for the ignition vacuum advance since it is a bleed off port.
However, it can be used if the bleed off hole/slot is closed.

The regular port for ignition vacuum advance is on drivers side, right above the idle mixture screw.
This port is for most units ported/timed, and for others it is a constant vacuum port, check location of slot, if below blade it is constant, if above it is ported.

Correct, there are some units that simply don't have a ported source for the vacuum advance.

[RWS]

Yes, sorry, I should have been more specific: my Q-Jet is a 1970 RA III carb, and it does not have ported vacuum, but I do appreciate some later carbs that came on engines with EGR systems do.

[Greg Reid]

Interesting, did not know that. I can learn something here every day.

Sent from my moto g(6) play using Tapatalk

[Kenth]

No 1970-74, except 1971 455H.O., Pontiac Quadrajet uses the "ordinary" ported source for the ignition vacuum advance above drivers side mixture screw.
Instead these engines uses the TCS system to control the vacuum advance.
No vacuum advance at idle, only with cold or to hot engine and when final transmission gear is used.

[Cliff R]

"Then I had the distributor vac advance hooked to a vac port in the back of the carb."

Any port on the back of the baseplate would have been manifold vacuum. There are no ported sources available on 1970 Pontiac carburetors that I am aware of. The rear of the baseplate has 1/8"NPT threads for various fittings used by the factory, all would be manifold vacuum.......Cliff

[Cliff R]

RPM has nothing to do with vacuum production falling off far as manifold or ported. Light engine load produces high vacuum. Vacuum changes or drops off quickly with increased throttle angle/engine load. The harder you load the engine the less vacuum is produced. At very heavy and full throttle the vacuum available above and below the throttle plates is pretty much the same, or zero.

ALL sources below the throttle plates will show basically the same readings and operate the vacuum advance in the same fashion.

This includes the "Tree" in the rear of the baseplate or any other source in the intake manifold.....Cliff

[Kenth]

I would avoid using a source on a single runner for the ignition vacuum advance, like the one on the #1 runner on a Pontiac intake, as the signal pulsates and are somewhat weaker than any plenum source under the carburetor throttle blades.