Mallory Distributor Vacuum Advance Canister Adjustment

[Scott Novak]

[74 Thing]

Does the can provide a certain amount of advance (ie from 8 degrees up to 15 depending on how the screw is set)?

[Scott Novak]

I don't remember how many degrees of advance the vacuum canister will provide. I think it may be 12°.

It takes a certain minimum amount of vacuum before the diaphragm will begin moving. Turing the adjuster counter clockwise will increase the minimum amount of vacuum required before the diaphragm begins to move.

When your engine is producing the maximum amount of vacuum from the vacuum port on the carb, you adjust the canister for the maximum amount of vacuum advance that you desire.

Scott Novak

[j-dub]

Scott,
Did you test the vacuum can with a vacuum pump? Do you know how low of vac pressure will start the can moving assuming the preload on the spring is not maxed out? More specifically can you make it start at 2-3 inches of vacuum?

Thanks,
Jeremy
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[Scott Novak]

I have taken the vacuum canister apart to see how it was constructed.

I am about to measure the actual vacuum versus degrees of advance. I need to pick up more vacuum hose and a few T fittings.

But I did in fact confirm that with the adjuster turned fully clockwise it takes less vacuum to actuate than when the adjuster is turned counter clockwise.

I also need to confirm whether my Weber 40 IDF provides enough vacuum to advance the distributor sufficiently.

HOWEVER, be aware that Mallory supposedly makes two versions of the vacuum canister and that one of them requires less vacuum to actuate.

Also, the vacuum canister on the Mallory Comp 9000 distributor that I am using connects to a pin staked to one of the outer holes on the rotating baseplate. So my results aren't going to be quite the same as the Mallory SVDAs most people are using where the vacuum canister is connected to a post staked to one of the inner holes of the rotating baseplate.

Imageshack lost a bunch of my photos including one showing the baseplate and the vacuum canister connection. I'll have to take some new photos. So far I've had NO response back from Imageshack about my photos, even though they claim to have them backed up 3 different ways.

Scott Novak

[j-dub]

Thanks Scott, Looking forward to your results.

For what is is worth my engine makes about 9" of vacuum so I lowered the vacuum spring pressure on my VJU4BR8 so it starts at about 2.5" vacuum and at 9" I have an additional 13 degrees advance.

I just googled Mallory Comp 9000, I see what you are talking about now and how different this is than the unilite. Interesting build. I have heard mixed reviews of the minimum amount of vacuum you need to get a unilite to start advancing, possibly people did not know they were adjustable or they adjust differently than yours.
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"POLICE STATION TOILET STOLEN ... Cops have nothing to go on."

[Scott Novak]

The Comp 9000 uses the same lower shaft and spring advance system as the Unilite. They don't make Comp 9000 distributors for for 4-cylinder engines so I had to do some part swapping and grind the extra 4 teeth off of the reluctor to make it work on my engine.

The rotating baseplate is basically the same as the Unilite except for some screw holes in different locations. Because the Comp 9000 distributor has a larger diameter bowl, they had to move post for the vacuum canister linkage to a hole on the outside of the rotating plate. You will see these extra holes in the Unilite baseplate. The baseplates are made for left or right hand rotation and small bowl or large bowl distributors. They just stake the post for the vacuum canister in the appropriate holes as needed.

9" of vacuum should be more than enough to actuate a Mallory vacuum canister. You just turn the adjuster counterclockwise to reduce the sensitivity to the correct level. But there is a point where turning the adjuster counter clockwise will compress the spring enough that it will begin to limit the maximum amount of advance.

If anyone talks about the minimum vacuum needed to actuate the vacuum canister, unless they specify that the adjuster is turned fully clockwise they don't know what they are talking about. Also, there are supposed to be two different versions of the vacuum canister with different sensitivities.

Scott Novak

[Scott Novak]

I measured the sensitivity of my Mallory Comp 9000 Vacuum advance canister tonight. Unfortunately the results are not as accurate as I would have liked. I discovered that my vacuum gauge has a sticky needle. I tapped the meter as I was taking measurements to reduce the errors as much as I could.

With the adjuster inside the vacuum canister fully clockwise, with somewhere between 4.5" Hg and 5.0" Hg to the vacuum advance diaphragm, the baseplate began to rotate and begin advancing the spark timing.

5" Hg = 0 Crank Degrees Vacuum Advance

6" Hg = 8.4 Crank Degrees Vacuum Advance

7" Hg = 11.2 Crank Degrees Vacuum Advance

8" Hg = 16.3 Crank Degrees Vacuum Advance

9" Hg = 17.3 Crank Degrees Vacuum Advance

10" Hg = 19.1 Crank Degrees Vacuum Advance

11" Hg = 20.6 Crank Degrees Vacuum Advance

12" Hg = 23.1 Crank Degrees Vacuum Advance

I don't think that my Weber 40 IDF's have anywhere near the amount of ported vacuum needed for the vacuum advance canisters. The Mallory vacuum advance canister really needs changeable preload springs inside and also a physical stop for maximum advance.

Scott Novak

[Scott Novak]

I'm probably going to have to break down and buy one of these Anti-Pulse valves:
http://vwparts.aircooled.net/Anti-Pulse-Valve-for-Vacuum-Advance-Distributors-p/anti-pulse-valve.htm

I think it's just a quick acting check valve with an air bleed across it. I actually tried making one of these today. It did help a little, but the check valve isn't sensitive enough.

I'm also going to open up another Mallory vacuum advance canister. I'll take some photos this time. I want to be able to replace the spring with a lighter one to make the advance more sensitive. There is a limit on how light a spring I can use. If the spring is too light it wont overcome the friction of the rotating baseplate.

It's also possible that I need to find a vacuum advance canister that is physically larger in diameter.

Scott Novak

[j-dub]

Did you measure how much vacuum your engine makes yet? I am guessing it makes somewhere between 8 and 10 inches and if that equates to 16 to 19 degrees advance you will already have to start limiting that down. Are you hoping it starts advancing sooner with the softer spring so your vacuum advance curve is softer, not so much like a step function?
_________________
1957 Oval rag
"POLICE STATION TOILET STOLEN ... Cops have nothing to go on."

[Scott Novak]

34 PICT puts out. about 8" Hg to the vacuum port. Unfortunately the 40 IDF takes it's vacuum from only one cylinder and provides about 2" Hg of vacuum, which is about 1/4 of 8" Hg which is close to what you might expect.

A weaker spring would allow the advance to start sooner. I'm not sure how I'll do it yet. I'll have to figure out something once I have the vacuum can opened up.

Scott Novak

[udidwht]

Pull from each IDF to a T. Also use the APV between the T and the distributor. This is how I'm set up and out on the highway I've seen vacuum as high as 6hg at times while cruising on the flats.

The APV will dampen any pulses in the line and help steady it. It's hit or miss with IDF's & vacuum.
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[raygreenwood]

[webebuggin]

The Mallory can is WAY to stiff,even with a mild cam, I used a can from a big cap cast iron distributor,in conjuction with the anti-pulse, t'd to both weber 44's, it pulls to 42 degrees, piks are in the gallery, The can was from a Bosch ZV/PAU 4r5 distributor, I added a return spring,and it works great,

[Tuna Tim]