RandyS

I was setting the timing on my 66 442, stock distributor with Pertronix. When I unhooked the vac advance line, (from the Edelbrock 750) I noticed no differrence in RPM. Normally it would go down, right? There is some vacuum being sucked at the port on the carb, but it doesn't seem like a lot. I figured that maybe the vac advance cannister was locked up. I pulled the distributor and tore it all down. The cannister arm has movement, but it seems pretty stiff. I bought a new cannister and it seems as stiff as the original. Are they normally pretty stiff to actuate?

redoldsman

They are pretty stiff. Did you put a hand vacuum pump on it to see if it worked?

Vintage Chief

Randy, did you purchase the adjustable vacuum canister? They make basically two models - non-adjustable & adjustable. The adjustable types, IIRC, have the octagonal shaped housing where the metal vacuum tube orifice is located (the non-adjustable do not have the octagonal shape). The instructions should indicate the maximum & minimum you can adjust the adjustable vacuum canisters. There should be an Allen Wrench opening where the orifice is located to adjust the vacuum canister. Check the instruction manual to see what it is set at from the manufacturer.

RandyS

No, I don't own one. I used the 2 lip version to create some suction, but it didn't move at all. It didn't leak however..........


No, I cheaped out and bought a non-adjustable one. The adjustment is for the amount of advance, not the vacuum needed to actuate it, right?

joe_padavano

Sounds like you are using ported vac at the carb, not manifold vac. Connect a vacuum gauge to that port and watch what it does as you rev the engine from idle to 2000.

Vintage Chief

Correct. The Allen Wrench (Key) adjusts the 'throw' of the actuator arm - full travel or less than full travel (increasing/decreasing advance).

oldcutlass

Incorrect, the adjustment is for the amount of vacuum it takes to actuate the arm. It should not be used to adjust the amount of advance. To adjust the amount of advance you should install a mechanical limiter.

Vintage Chief

Are you sure about this?

oldcutlass

Positive, the screw adjusts the spring tension within the canister to adapt it to variations in available vacuum to fully deploy the arm.

Vintage Chief

OK Agree. I think I explained it incorrectly. Yes, you tighten and loosen the spring inside the diaphragm of the canister, right? The more you tighten the spring the later the advance the more you loosen the spring the earlier the advance, right? But, I still have a question. The vacuum being applied is going to remain the same is it not? The only delta is the spring's ability to allow the actuator rod throw to extend earlier (and full travel) as opposed to a tighter spring which would make the advance come in later (and less travel). Am I right about this? The vacuum remains the same, it's just when the advance comes in based upon spring tension, isn't it?

RandyS

It sure can be confusing using the google machine. While I was waiting for this to be worked thru, I went online and did a search about adjustable cans. I actually found both versions of the results of adjusting the cans. I think the consensus was that Eric's version is correct, that the adjustable cans vary the amount of vacuum needed for a certain amount of advance. The total amount of advance is limited by mechanical means outside of the cannister.......... back to work ............

Vintage Chief

Kind of a good question for me (at least), because I've never considered it from the point of the 'amount' of vacuum being applied. I can see in your discussion, if you have less than optimal vacuum, you could loosen the spring tension based upon a low vacuum you are pulling, I guess. On the other hand, if you had a high vacuum, you could tighten the spring tension. I've never thought it in this sense. I just always considered whatever vacuum you had the vacuum would remain constant and nearly the same for many vehicles. Interesting.

Vintage Chief

Ah, I think I figured it out. Even if vacuum remains the same, it's the 'rate', at which the advance comes in. Less tension on the spring means the advance comes in earlier at a lower vacuum. Kind of hard talking about it since I was thinking the vacuum itself from the hose remains the same (which it most likely does) but the 'amount' of vacuum required is lower when the spring has less tension. Argh. Tough one to think threw for my shallow mind.

joe_padavano

joe_padavano

Here's another one.

oldcutlass

You'll find that it does not work that way Joe especially if your using one on low manifold vacuum with a radical cam. The advance will be very erratic, the mechanical stop is the only way to go.

joe_padavano

This is a spring, a vacuum diaphragm, and an adjustable stop. It works exactly that way ASSUMING the vacuum source is stable. Obviously a radical cam doesn't provide a stable vacuum source (which is why W-30 carbs didn't have power pistons). The quality of the vacuum (or lack thereof) doesn't change the physics of how the adjustable mechanism works.

OLDSter Ralph

Actually, there were three different power piston springs available for different "applications" from Rochester. I don't remember where I got the Rochester power piston spring numbers, could have been from Doug Roe's book. I don't remember seeing anything in stuff that came from Oldsmobile.
One spring was available for as low as 8" of vacuum. With a longer duration cam, you can have stable vacuum, you just don't have much of it. Thats why power brakes "didn't work well" with the 328/328 cam. The 68 H/O (W-45 non A/C) were kinda borderline with the 308/308 degree cam.
......Just my two cents worth.

oldcutlass

I believe there is no stop in there, just a spring. It's a rate adjustment not a limit. At least all the ones I've played with.

cfair

This may not help, but I mostly eliminated part-throttle ping by changing to different, Non-Olds, fixed vacuum canisters in my HEI. See the article by Lars Grimsrud that shows the degrees advance for each canister and vacuum settings. The adjustable ones, having tried many settings, just didn’t work for me - they all pinged mid-throttle on a hot day. But we have cr*p gas in CA these days too, so I’m sorta chasing my tail.

If you’re pinging, you’ve got too much advance too soon, which means you want an canister that gives less advance and/or later in the RPM range.

Note that distributor advance degrees are exactly half of crankshaft advance degrees, so be ready to understand that 6 on the dizzy means 12 on the crank & so on. This is confusing, don’t forget to account for it.

Hope that helps. It was an eye opener to me that there were so many different canisters for the HEI distributor. It was really just a matter of getting a few and trying them. I’m not sure I’m at optimal yet, but the cars sure ping a lot less.

cheers
cf

joe_padavano

Look at the chart. The rate is the slope of the LH part of the curves, and it is the same for all adjustments. The only difference from turning the screw is the limit on degrees of advance. That means that the screw is changing where the diaphram stops at max vacuum.