http://heartland.chevelles.net/RCstorage/qjet_labels.jpg

My question is about vacuum advance on my basically stock SB 400. While I already know the idle air bleeds need to be opened up a bit to make this thing idle better, my question now is about the vacuum advance. The source I'm currently using for vacuum advance does not appear to have any vacuum at idle, though full vacuum and mechanical advance is all in by 3000 RPM. If I temporarily connect the vacuum advance to the fitting on the passenger side of the carb, which appears to be manifold vacuum, the timing and therefore the idle picks up noticeably.

The only problem currently is that the fitting on the side is very small and I'd have to make an adaptor so the larger vacuum line hose would fit securely to it. My question is, however, am I correct in my belief that the fitting at the front is ported vacuum and that I should find a way somehow to connect to manifold vacuum for the vacuum advance? The fitting I am currently using is the fitting that was used in stock configuration when this engine/carb were in a '76 Chevy Blazer. I suppose a seriously smog-era engine would have been using ported vacuum.

Other than the PCV outlet, there do not appear to be any other vacuum outlets on this carb.

TIA.

Use the one on the pass side, and yes it will gain rpm's it the vacuum advance working.
Here's some links that will help you with your Q-Jet
----> http://www.florida4x4.com/tech/quadrajet/

Pat

Rich, you have got it right on the vacuum ports. Personally I usually end up preferring ported vac. I have run into many issues with manifold vacuum to the advance. If you do choose to run it that way set the timing with hose disconnected. when you plug it in the idle speed will jump up, you will need to re-adjust the carb.

BTW, it's not bigger idle air bleeds you need, it's bigger idle fuel feeds These are usually called the idle feed restrictions. yours are probably in the mid .030s, I would drill them to either .039 or .041. this would make more total fuel available to the idle and transition circuits. Sometimes this leads to an amazing improvement in around-town driveability.

Rich,

Do you have a vac fitting in one of the runners on the back of the manifold - say for, perhaps, the power brakes? If so you could easily replace that fitting with another that has an additional port. Check the Gound Up catalog - they have a lot of such fittings.

Dan

Personally I usually end up preferring ported vac. I have run into many issues with manifold vacuum to the advance.

What kind of issue could arise by using manifold vacuum? I'd hate to trade one set of issues for others.... My other cars use manifold vacuum and I've never had any idle problems other than those caused by the cam!

A factory hei unit requires using tha exact port you have it hooked up to in the picture(ported vacum) however it depends on your distributor setup. Probably should have between 10-14 degrees advance at idle and should advance to around 30-35 at 3500 rpm.To adjust the carb always use a simple vacum gauge. Adjust the idle screws until you get the most vacum. A good engine makes around 19-21 adjusted right at 750rpm. Taking into consideration stock (no huge cam) an engine makes the most vacum it can slowing down under engine braking which if you had your ditributor hooked up to amnifold vacum it would fully adnavce when it wasnt needed. So thats another reason why stock is hooked up that way. It will only advance when the throttle is opened.

stock HEI's were only hooked to ported vacuum for some stupid mid 70's emmissions logic. any pre-egr car had it hooked to manifold vacuum from the factory.
i've yet to have a car that runs better on ported than manifold vacuum.
the only "issues" one might have with using manifold vacuum instead of ported is better drivability and increased gas mileage.

You can't say every engine will run better with either manifold or ported vacuum. There are too many different combos that will work better with one but not the other.