[dataway]

Is the carb stud on a Q-Jet a suitable source for manifold vacuum for ignition advance?

[25stevem]

Are we talking on a stock duel plane intake?

[Schurkey]

Originally Posted by dataway

Is the carb stud on a Q-Jet a suitable source for manifold vacuum for ignition advance?

The threaded stud--usually with wing-nut--that holds the air cleaner lid?

No, that's not my first choice.

Although I used to use it to seal the distributor end of the vacuum-advance vacuum hose when setting timing.

[OG68]

This one. Used on 68 Manual Transmissions. Possibly others.

[Schurkey]

I'd expect that to be manifold vacuum. Test it and see.

If so, and you want manifold vacuum for your vacuum advance, that'd be fine.



That's not a vacuum source I'm familiar with. Is there a missing nut on that stud to hold the carb base against the gasket?

[dataway]

Yes the one in the photo is missing the nut.

Basically it's just a hollow stud that goes into that intake runner.

[Tom Vaught]

Normally Vacuum Advance gets a balanced vacuum from both sides of the carb where it mounts on the divided wall of the carb. If you put it on a carb stud you get one half of the signal to the vacuum advance. Suction, space, suction, space, suction, space.

At the back of the carb, you get signal from BOTH intake manifold "Planes" and a continuous signal to the vacuum can.
There is also a fitting on a stock Pontiac dual plane intake that "reads" vacuum signal on both planes.

Tom V.

[dataway]

Thanks Tom, exactly what I wanted to know. I have numerous options for a source, this one was just the easiest.

[Schurkey]

What did GM use that vacuum fitting for?

[dataway]

As OG68 said, it was used on 68s with the five port TVS, spark retard system and manual trans ... don't think I've ever seen it used anywhere else.

Anyone know what the vacuum port on to the right of the fuel inlet on this 1971 704 carb is for? And if it would be suitable for vacuum signal to the advance can?

[25stevem]

That's a ported vacuum source.
There's always the option of putting a Y or T into the Choke pull off line since that is aways full Manifold vacuum.

[Cliff R]

Source location is too high for ported vacuum advance at that fitting.......

[Matt Meaney]

if it's in the main body of the carb, it goes to the vacuum modulator on an auto trans.

[dataway]

Talking about the port to the RIGHT of the inlet in the photo. The modulator port is on the left with the sealing barb.

This port is in the base plate. Anyone know what it was used for on a 71 Pontiac? Carb is a 7041264

[Kenth]

It´s used for the coal canister (evaporation control system).

[25stevem]

Bingo!
If I recall right the bigger size hose that would be needed is the tip off, no?

[dataway]

Starting to look like the best source may be the large port on the rear ... I was only wanting to avoid that because it also supplies the power brake, AC system, and headlights .. didn't want my timing changing when I hit the brakes Although I have no idea if that would happen or not.

[Tom Vaught]

There is a check valve in the Power Brake can that keeps the issue you describe from happening.

Tom V.

[Matt Meaney]

Originally Posted by dataway

Talking about the port to the RIGHT of the inlet in the photo. The modulator port is on the left with the sealing barb.

]

oops, i was thinking right as in passenger side of the car.

My '71 Gp had a tree at the back of the carb with several ports on it. one went to the brake booster and another to the vacuum advance. never noticed it do anything funky while driving .

[dataway]

Matt ... figured you thought that ... I should have specified Pas/Drv side

Since I'm using a 71 Q-Jet I'm guessing that solution will work for me too.

Engine has 20+ inches of vacuum at idle ... so there's no shortage