Powerglide modulator vacuum source

[Doug W.]

I've restored my engine and had my Powerglide transmission rebuilt. Everything works great, except the Powerglide downshifts are bone-jarringly hard. The trans guy says the vacuum source for the modulator should be manifold (intake) vacuum, NOT ported vacuum. Other web sites on Powerglides appear to confirm this.

The 1963 assembly manual (M35, sheet 4) shows the modulator line having a "T" on the engine end, which is connected to the distributor vacuum advance, with the straight end of the modulator line being connected to the front of the Carter AFB carburetor. This is exactly how I have the modulator, distributor advance and carburetor lines connected.

Questions: Assuming the distributor vacuum advance wants ported vacuum, and the Powerglide modulator wants manifold vacuum, how can both be connected to the same line, as shown in the 63 Assembly Manual?

Is the front port on the Carter AFB ported vacuum or manifold? My guess is ported, since it is connected to the Distributor vacuum advance. but I can check this easy enough with my vacuum gauge tonight.

Is it possible that either the Corvette Powerglide modulator and/or the Corvette distributor vacuum advance, are different? Could it be that the Corvette Powerglide modulator works off ported vacuum or the Corvette distributor vacuum advance works off manifold vacuum?

Any other theories on how the trans guy and the assembly manual can both be right, or advice on which is wrong?

Thank you!

Doug

[John H.]

Re: Powerglide modulator vacuum source

Is it possible that either the Corvette Powerglide modulator and/or the Corvette distributor vacuum advance, are different? Could it be that the Corvette Powerglide modulator works off ported vacuum or the Corvette distributor vacuum advance works off manifold vacuum?

Doug -

Both the Powerglide modulator and the distributor vacuum advance require full manifold vacuum. There was no application for "ported" vacuum on carbureted '63 Corvettes.

[Doug W.]

Re: Powerglide modulator vacuum source

Thank you, John!

Last night I checked the vacuum and got about 18 inches at idle, which tells me my assumption about this line being ported vacuum, was incorrect. It does appear to be manifold vacuum, as you and the trans guy have indicated.

The trans guy wants to see the car next week, so hopefully we can get that sorted out with him. He said there's an adjustment on the modulator that might smooth out the downshifts.

But this brings up another issue for the distributor vacuum advance. When attaching this same vacuum source to the distributor vacuum advance, my timing increases by about 10+ degrees, at idle. Now that I know this connection is manifold vacuum, not ported vacuum, this sort of makes sense. But my question is, is this correct? In other words, should the distributor timing increase by 10+ degrees when connecting the vacuum advance line?

The trans guy sort of indicated that perhaps there are different vacuum advance mechanisms that are designed to work from manifold vacuum. If this is correct, does it appear like I do not have the right vacuum advance can on my distributor?

Thank you!

Doug

[John H.]

Re: Powerglide modulator vacuum source

But this brings up another issue for the distributor vacuum advance. When attaching this same vacuum source to the distributor vacuum advance, my timing increases by about 10+ degrees, at idle. Now that I know this connection is manifold vacuum, not ported vacuum, this sort of makes sense. But my question is, is this correct? In other words, should the distributor timing increase by 10+ degrees when connecting the vacuum advance line?

The trans guy sort of indicated that perhaps there are different vacuum advance mechanisms that are designed to work from manifold vacuum. If this is correct, does it appear like I do not have the right vacuum advance can on my distributor?

Thank you!

Doug

Doug -

What you're seeing is exactly the way it's supposed to operate; if you look closely at the bracket that attaches the vacuum advance unit to the distributor, you should see "201" stamped on it if the vacuum advance is original to the car.