YAVQ - Yet Another Vacuum Question

[Shannon Burgess]

I pulled the vacuum advance hose off the carb on my '69 L-46 today to check how much vacuum was getting to the advance can at idle. Much to my shock, I only got a reading of about 3" of vacuum at 700rpm. As I increased the engine speed, the vacuum increase as well. I switched the vacuum gauge over to the vacuum fitting on the manifold and was able to get more realistic readings of 15" or so.

Am I missing something here? The car idles well, doesn't overheat, etc.

Thanks,
Shannon

[Joe C.]

Re: YAVQ - Yet Another Vacuum Question

Shannon:

Sounds like your distributor can is using ported vacuum. As I understand it, ported vac was used on many big blocks post 1967, in anticipation of the 1970 Clean Air Act. I would use manifold vac instead of ported, despite how the engine was designed, but I would take the advice of those who have empirical, rather than subjective evidence.
The proof would be, that if you switch to manifold vac, and the engine does not detonate under load, then keep it there. Fact is, the engine will perform at it's best with maximum advance, just prior to detonation.

Joe

[John H.]

Re: YAVQ - Yet Another Vacuum Question

Shannon -

Ported vacuum was an early emissions strategy. It will have more stable idle and will run cooler at idle and low speed with manifold vacuum. Just re-set idle speed and touch-up your idle mixture screws and you'll be in business. Your vacuum advance can supplies 15 degrees advance at 12" Hg., so it will be fully deployed at idle.

[Shannon Burgess]

Re: YAVQ - Yet Another Vacuum Question

Thanks guys. So what should I do? Just block off the current port on the carb? Can I tie the vacuum advance line inline with the PCV or should I use the direct manifold port that's supplying vacuum to the headlight/wiper door system? Is there another port on my 7029207 Quadrajet that will supply manifold vacuum?

Thanks,
Shannon

[Jeffrey S.]

Re: YAVQ - Yet Another Vacuum Question

Shannon:
The vacuum advance is a ported vacuum. You need to be off idle to get vacuum out of the front opening. I took the vacuum off the AIR fitting (AIR system was removed from the car and the fitting capped). I had put a different cam in and was getting a lope at idle until I noticed that as I advanced the timing the engine smoothed out. By using a non-ported source I was able to get significantly more advance at idle and the engine smoothed out beautifully. My carb is #7029202 which is identical to yours. It is on a 69 small block.Hope this helps.
Jeff

[John H.]

Re: YAVQ - Yet Another Vacuum Question

Shannon -

Just cap off the ported vacuum source; you can tap into the fitting on the manifold, or you can tee into the hose that feeds the vacuum break diaphragm. I'm not that familiar with Q-Jets, but the vacuum break on a Holley gets full manifold vacuum - check it with your vacuum gauge.

[Shannon Burgess]

Re: YAVQ - Yet Another Vacuum Question

Jeff, I think I know where the AIR port is on the carb, but could you tell me so I know for sure. I seem to remember a cap over a certain port off the rear side of the carb, but confirmation from someone who's "been there, done that" sure helps!

Shannon

[Jeffrey S.]

Re: YAVQ - Yet Another Vacuum Question

Shannon:
The AIR port is on the passenger side of the carb going straight to the side. It should have a tube coming out just behind the choke pull off assembly. Simply attach a hose from here to the advance can. Be sure to cap off the original port on the front of the carb so that you don't have a vacuum leak when you are off idle. I left the metal vacuum advance tube and hose in place and put a golf tee in the open end of the hose so that when I show the car I can hook it up properly and then unhook it for driving. Hope this helps.
Jeff