Where does my distributor vacuum go?

[71swb4x4]

Does it hook up to the capped port on the front of my carb? The screwdrivere is pointing to the port I am referring to. This is the only vacuum port I can find on the carb.



Click on the picture for a bigger version.

[Stocker]

Pretty sure that's where mine is -- not easy to check right now with several inches of wet snow on the hood...

[Slammed70]

Anywhere theres manifold vaccum

[poorboy]

You want it hooked to a port that increases the vac as the rpm's go up. If you hook it to a manifold vac port it will be at constant full advance in the timing. Just start the motor and use a vac gauge or hold your finger over the port. Raise the rpm it should increase the vac as you raise the rpm.

[DavesRide]

Originally Posted by poorboy

You want it hooked to a port that increases the vac as the rpm's go up. If you hook it to a manifold vac port it will be at constant full advance in the timing. Just start the motor and use a vac gauge or hold your finger over the port. Raise the rpm it should increase the vac as you raise the rpm.

Go with poorboy's advice. The only port that has vacumm during acceleration it above the throttle plate. Look for a port above throttle plates and hook it up to the distributor vac advance. I know the ones below have no vac on accel from my experience with old truck wipers. Rememberwhen?

[hdflstf]

None of the above is entirely correct.

What you need is to hook up to the vacuum post that DOES NOT (assuming everything is set up properly) have vacuum at idle. Once the throttle is opened slightly, the port will have vacuum. From there, the vacuum will be engine vacuum which changes depending on throttle position and engine load. The port you are pointing to looks to be the right place. You do not want straight manifold vacuum as you will have a hard time setting and keeping a consistent idle speed.

[WorkinLonghorn]

Yes (to your question).

[VetteVet]

Originally Posted by hdflstf

None of the above is entirely correct.

What you need is to hook up to the vacuum post that DOES NOT (assuming everything is set up properly) have vacuum at idle. Once the throttle is opened slightly, the port will have vacuum. From there, the vacuum will be engine vacuum which changes depending on throttle position and engine load. The port you are pointing to looks to be the right place. You do not want straight manifold vacuum as you will have a hard time setting and keeping a consistent idle speed.

What you have posted is correct except that the OP did not indicate whether he has an HEI distributor or the stock points system. The vacuum advance is designed for full manifold vacuum at idle. The diaphragms are different between the two and the normal hookup for the points distributor is at a tee on the rear of the manifold behind the carb. IT shares a connection with the brake booster and the trans vacuum modulator.

It appears from the picture that he does have an HEI distributor, if so then my post is for informational purposes only.
There are tuners that recommend using full manifold vacuum in all cases and claim better performance. I can not substantiate that so I will defer to the experts.

[Billla]

Originally Posted by poorboy

You want it hooked to a port that increases the vac as the rpm's go up. If you hook it to a manifold vac port it will be at constant full advance in the timing. Just start the motor and use a vac gauge or hold your finger over the port. Raise the rpm it should increase the vac as you raise the rpm.

Sorry, this is just not correct. Vacuum typically decreases as RPM increases. At WOT if the carb is sized correctly there should be no vacuum

The simple answer to the question is that if you have a stock or near-stock cam, you should connect to ported vacuum as no additional vacuum advance is needed at idle. Ported vacuum is taken above the throttle butterflies, and so is less than manifold vacuum at idle.

If you have an aftermarket cam that's a little warmer, connecting the distributor advance to manifold vacuum will provide additional vacuum advance at idle which can improve idle quality.

Once the butterflies are open to any extent, manifold vacuum and ported vacuum will be virtually identical and so there is no performance/fuel economy/etc. difference - the only reason for additional advance at idle is to smooth out a lumpy cam.

BTW, this has been hashed out a million times - search is your friend

[motopatzo]

I'm gonna have to agree with Billla

[hdflstf]

Originally Posted by VetteVet

What you have posted is correct except that the OP did not indicate whether he has an HEI distributor or the stock points system. The vacuum advance is designed for full manifold vacuum at idle. The diaphragms are different between the two and the normal hookup for the points distributor is at a tee on the rear of the manifold behind the carb. IT shares a connection with the brake booster and the trans vacuum modulator.

It appears from the picture that he does have an HEI distributor, if so then my post is for informational purposes only.
There are tuners that recommend using full manifold vacuum in all cases and claim better performance. I can not substantiate that so I will defer to the experts.

Did not look at the HEI. Of course I was thinking along stock not modified.

I would expect that the full vacuum at idle would be in conjunction with all the smog controls and equipment that would be installed on the engine that the HEI would have originally been removed from.

Since the engine looks to be more like a non-smog era type, I would still run the vac advance to the ported vacuum so it would operate as a stock type distributer would.

[ChevyC102007]

I have the vaccum advance hose connected to the vaccum port located underneath the fuel inlet line on the left hand side of the carb. As stated this is the ported vaccum for the Rochester QJet carb (I.e no vaccum at idle but pressure increases with engine RPM's) from my research this is where it is on as near as I can tell all stock engines with Qjet carbs until the middle 80's when the switch to TBI tock place. The PO of the truck had the hose hooked to a constant vaccum sorce on the carb causing absolutely NO advance at higher RPM's. Just my 2 cents.

[Huck]

Kind of depends on the model of HEI, cam, carb. In any case you set your timing with the vac line bocked off --not connected to the distributor. If you time it to 8 deg advance - you may or may not have adequate idle so I've had my best results by setting initial timing, then with a vacuum gauge, hood up the vac advance to manifold vacuum and then refine the timing to maximize Vacuum at the gauge --also may need to tweek the idle mix settings. This has worked for me in every case. ___See GMC Pauls tech tips on timing via Vacuum____Huck