C2 Vac Advance Cans - design / deterioration

[Wayne M.]

There's been a lot of discussion on selecting the correct can, and the fact that the spec's indicated by the part # are rarely met in fact.

What are the design parameters ? I'm thinking a single diaphragm, but with various springs, various (internal ?) stops.

Looking at these units, the slot where the rod attaches to the advance plate seem to be identical. The rubber cushions (?) that contact slot ends also seem constant.

I've seen NOS in-the-box units offered on eBay WITHOUT the rubber sleeve. Won't that affect the travel (ie. total advance ) ? If this is a factor and the rubber deteriorates over time, won't the timing change ?

[Stuart F.]

Re: C2 Vac Advance Cans - design / deterioration

Wayne;

That's quite a collection you have there. I have rarely seen a sleeve on the rod, and certainly it would affect the total advance movement of the VAC. The only one I have in my stock with a sleeve is for an HEI distributor. I have seen different guide slots, although slight. That may just be a brand thing or QC. I, of late, have seen more rods that are twisted on new repros which causes binding in the slot and the hole in the advance plate (not good). I have straightened a few using two (2) vice grips - one to grip the rod firmly about an inch from the diaphragm and another to twist it at the bend in such a way that you twist the rod only and do not apply torque to the diaphragm.

The bottom line, as our experts will tell you, is to test each VAC to see that it is close to the advertised specs that YOU need for your application. Many repros of late are mis-stamped with popular numbers that may look good to judges, but won't give you the performance characteristics that YOU need. Start by testing with a Mityvac to see that the movement begins and is all in at specs. Then, if you have access to a distributor machine, set it up in a distributor and test for the advance degrees, and for a more accurate look at it's movement.

Stu Fox

[Joe R.]

Re: C2 Vac Advance Cans - design / deterioration

Hi Wayne:

Based on my tests with a Mityvac and the advance can on my car, the rubber sleeve does in fact limit the maximum travel and hence, the maximum advance. It surprises me that GM would use such a fragile part to control such an important parameter. These sleeves often deteriorate and fall off.

[John H.]

Re: C2 Vac Advance Cans - design / deterioration

Hi Wayne:

Based on my tests with a Mityvac and the advance can on my car, the rubber sleeve does in fact limit the maximum travel and hence, the maximum advance. It surprises me that GM would use such a fragile part to control such an important parameter. These sleeves often deteriorate and fall off.

Joe -

Millions of Delco distributors used rubber sleeves on the limit pin on the autocam to limit centrifugal advance - and, yes, they dried up and fell off.

[Stuart F.]

Re: C2 Vac Advance Cans - design / deterioration

Back in the day we used to change out sleeves on limit pins with other more durable materials of equal or varying sizes (depending on the limit desired), and there actually were commercially availalble kits for that purpose complete with small lock rings. Working with a stock engine configuration; however, there isn't a whole lot of "tweaking" one could or should do in this area so if the sleeve passes inspection and your advance map meets specs you leave it alone.

Stu fox

[Steven H.]

Re: C2 Vac Advance Cans - design / deterioration

I just replaced my defective vacuum advance can with a new one but since I didn't see any rubber sleeve on the old one, I removed it.
Can I run it this way (without the sleeve)or would it be better if I reinstalled it

[Duke W.]

Re: C2 Vac Advance Cans - design / deterioration

What's the mfg/pn. What are the specs? Does it meet specs with the bushing? Without?

How do you expect anyone here to answer your question without knowing the above and testing it.

Duke

[Steven H.]

Re: C2 Vac Advance Cans - design / deterioration

What's the mfg/pn. What are the specs? Does it meet specs with the bushing? Without?

How do you expect anyone here to answer your question without knowing the above and testing it.

Duke

Sorry about the lack of info, that would be helpful.
New part number is NAPA Echlin VC680, suppose to be the correct replacement for a '64 327/300. I don't know if it meets the specs with or without the bushing. My old one did not have one on it but it was running good until the diaphram inside it failed. After reading the above posts it looks like there should be one on it and my old one just rotted away.
I will put the bushing back in and see what the advance specs are. Then I will have a starting point.
Trying to trace down an overheating problem. Advance can did help some but now I see that my radiator has developed problems of its own, leaks at a couple of seams and won't hold pressure. Better get this fixed first.

[Duke W.]

Re: C2 Vac Advance Cans - design / deterioration

That thing is a boat anchor!

Manual trans or Powerglide?

[Alan D.]

Re: C2 Vac Advance Cans - design / deterioration

Know I have a used FI distributor (acquired in early 70's) and it sitll has rubber, a new unit still in a red Delco box has the rubber, and my driver has always had the rubber. FYI

[Stuart F.]

Re: C2 Vac Advance Cans - design / deterioration

Alan;

Sounds like you're all set to go out on the town.

Now, we are talking about the VAC, right? We did get off onto the limit pin of the mechanical advance mechanism on to which I have almost always seen a sleeve or bushing, if you will. On the VAC's, however, of late I have not seen many with "rubber" sleeves, that's not to say they shouldn't be there. My latest #236 which is supposed to provide 16 degrees advance, does so on a distributor machine w/o a sleeve. If it had a sleeve it would not do that. I suspect the repros are being made to meet the spec w/o a sleeve to save some cost. Also, since I have seen several that needed to have their stems "twisted" in order to work properly, perhaps the QC is such that a sleeve would only exasperate the problem. What do you think?

Stu Fox

[Alan D.]

Re: C2 Vac Advance Cans - design / deterioration

Stuart, if "on a distributor machine w/o a sleeve" gives you the right advance then its a done deal. Perhaps you're right about a cost savings, but not sure why GM did not see that? Hope some pro's chime in.