Vacuum Advance on 70 corvette with TI distributor

[Alfred W.]

I want to install a new vacuum advance canister on a TI distributor without taking the distributor out. When the canister is installed in the distributor, the vacuum advance pin that goes into the plate has a huge amount of slop and will not advance the
distributor. There is a small rubber spacer on the vacuum advance pin but with it in place there is too much slop for the advance to work. Can anyone shed some light on this issue? Thanks for any help.

[Duke W.]

Re: Vacuum Advance on 70 corvette with TI distributor

The VAC can usually be changed with the distributor in the car by using a Mity Vac to pump it down to the limit of pin travel and then carefully working out the pin.

A little play in the pin hole is not uncommon, but you are saying that it is so sloppy that pumpiing the VAC down doesn't even rotate the pole piece? That would be very unusual. The VAC does not rotate the distributor. It just rotates the breaker plate (points) or the stationary pole piece (TI).

It would really help if you state the engine, transmission, ID stamped on the VAC and whether or not it is set up with the OE TCS or has been converted to full time vacuum advance.

It's very difficult to diagnose the problem with so little information.

Duke

[Alfred W.]

Re: Vacuum Advance on 70 corvette with TI distributor

Thanks Duke,
The car is a 70 LT1 with TI distributor, 4 speed trans, and is set up with OE TCS. The original vacuum can is stamped MS 201 15 and I replaced it with Eichlin VC680 Stamped B!. My question is why is the hole in the breaker plate sooooo large in relation to the pin on the vacuum canister?

[Joe L.]

Re: Vacuum Advance on 70 corvette with TI distributor

Thanks Duke,
The car is a 70 LT1 with TI distributor, 4 speed trans, and is set up with OE TCS. The original vacuum can is stamped MS 201 15 and I replaced it with Eichlin VC680 Stamped B!. My question is why is the hole in the breaker plate sooooo large in relation to the pin on the vacuum canister?

Alfred-------

All I can assume is that someone modified the plate. The hole in the plate should be right at 0.155" in diameter. The rod on the vacuum control should be right at 0.153". So, there should be very little clearance.

[Duke W.]

Re: Vacuum Advance on 70 corvette with TI distributor

The B1 is a poor choice because it may take up to 19" Hg to pull it to the limit, but GM inappropriately substituted a VAC with this spec for the ...201 decades ago as a service part even after the ...201 continued to be used on some production engines, and the aftermarket followed suit. Many regular TDB members are aware of this problem as it's been discussed many times.

The best match for the ...201 is the 15" B22. If the TCS is bypassed to allow full time vacuum advance, the 8" B28 should be installed.

I have no idea why the pole piece base has such a large hole, but you are going to have to remove and disassemble the distributor. The hole in the pole piece base plate was originally punched, which left the upset metal protruding. It may have then been reamed to spec.

It may be possible to peen the upset metal down and then ream it to spec or the pole piece may have to be replaced. It may also be possible to install the pole piece on a good base plate, but I'm not sure. Without actually seeing the problem, it's tough to determine.

You should consult the distributor rebuild specialists.

Duke

[Alfred W.]

Re: Vacuum Advance on 70 corvette with TI distributor

Thanks Duke,
I used the VC680 stamped B1 on the advice of Lars Grimsrud since it was very extensive. I will take you advice and install a B22.
Yes I am going to pull the distributor and check to pole piece base. Again, Thanks for your advice and help. Al Wendland

[Duke W.]

Re: Vacuum Advance on 70 corvette with TI distributor

I'm surprised Lars would recommend a B1. We are usually on the same page, but then with the TCS fully functional the difference between a 19" and 15" VAC may not be noticeable.

When you get to the point where you can snap a clear photo of the base plate hole for the VAC link, dig up this thread and post it, and also send it to Lars for comment.

Duke

[Alfred W.]

Re: Vacuum Advance on 70 corvette with TI distributor

Thanks again Duke,
I figured out my problem after I removed the distributor from the car. The vacuum plate was rotated and the howl for the vacuum canister was
rotated out of sight until the distributor could be disassembled to expose the correct hole in the bottom plate. Just takes some time. Thanks again Al

[Duke W.]

Re: Vacuum Advance on 70 corvette with TI distributor

So you're saying that the VAC link was not engaged with the stationary pole piece base plate (not "vacuum plate" as you say). That would mean that the stationary pole piece was unconstrained from rotating in which case I can't imagine that the engine would even start and run because the ignition advance was not under control. It could also damage the wires from the pole piece.

Duke

[Wayne M.]

Re: Vacuum Advance on 70 corvette with TI distributor

Here's a photo to help us follow the discussion (which I find interesting ). The hole for the vac adv arm is on a slightly smaller radius from the big shaft hole than the 3 other holes that mount the magnet.

[Joe L.]

Re: Vacuum Advance on 70 corvette with TI distributor

Here's a photo to help us follow the discussion (which I find interesting ). The hole for the vac adv arm is on a slightly smaller radius from the big shaft hole than the 3 other holes that mount the magnet.

Wayne------


This is not the pole piece used for 1970 LT-1. However, I believe the base plate is the same.

[Wayne M.]

Re: Vacuum Advance on 70 corvette with TI distributor

Joe (and others) -- just learned something new. As you say, my 1964276 base plate - stationary pole piece was not used for the 1970 LT-1 (which is #1960779). Nor any other distributors with the exception as follows: Looking at my P&A catalogs from July '65 to Jan '70, it seems that the "276" is for '64 and '65 fuel injected distributors; specifically the 1111064. All other T.I. distribs use the "779".

I compared the two side-by-side and found that the pick-up wires (siamesed, with a pale green stripe on one) on the "276" are about 1_1/2 inches longer than the "779". I wonder if this has something to do with the Fuel Injected distribs standing taller than the regular carb units, and need the longer wires to feed down and under the distributor ignition shield ?

I believe the repro catalogs offer only one version.

Opinions welcome