Correct vacuum signal for a stock 69 440

[Sixt8Chrgr]

My car has a aftermarket lopey camshaft, and I am only getting 6* vacuum from the vacuum advance port on my stock AVS carb. The car is idling at 900 rpms and timing is set at 15*. What is normal with a stock camshaft?

[BSB67]

Don't measure the vacuum port if you are trying to determine engine vacuum

[XH29N0G]

Don't measure the vacuum port if you are trying to determine engine vacuum

Can you explain why?  I have been measuring using the unported vacuum nib on the carburetor and do not have a manifold port that I am aware of (aftermarket manifold).  Just curious. 

[Sixt8Chrgr]

Should I be measuring engine vacuum from the tree on the rear of the manifold?

[Back N Black]

There are two types of vacuum, ported and manifold vac. Ported vac is pretty much non-existent when the throttle blades are closed, normally you vac advance is connected to ported vac. As you accelerate the throttle blades open creating vac and adjusting your timing. Manifold vac works opposite, when engine is at idle manifold vac is at max, when you accelerate manifold vac drops. If you want to measure your engine vac it has to be at the manifold port.

[Sixt8Chrgr]

There are two types of vacuum, ported and manifold vac. Ported vac is pretty much non-existent when the throttle blades are closed, normally you vac advance is connected to ported vac. As you accelerate the throttle blades open creating vac and adjusting your timing. Manifold vac works opposite, when engine is at idle manifold vac is at max, when you accelerate manifold vac drops. If you want to measure your engine vac it has to be at the manifold port.

Thanks

[green69rt]

Just to be clear, ported vacuum is measured above the throttle plates and manifold vacuum below, or is it something else?

[XH29N0G]

Just to be clear, ported vacuum is measured above the throttle plates and manifold vacuum below, or is it something else?

Yes.  Mine has a little hole in the throttle body that connects up to the port.  The hole is about the height of the top of the transition slots and in one of the front barrels.  May be different on other carburetors.  The blades open up over the port, I do not know if that is important for controlling how it behaves.  What I see is no vacuum at idle, high vacuum at cruise, and low vacuum at WOT.  I hooked up a vac gauge out of curiosity to see what it was doing and do not know if this is the only way it is supposed to behave.

[John_Kunkel]

In theory, there should be no vacuum at the ported nipple at idle but lumpy cams usually require the throttle blades to be open more to maintain idle speed and, as mentioned above, this exposes the transition slots which creates vacuum in the ported nipple.