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boilermaster

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Hey Mustang owners,

I have a question about vacuum advance on our distributors,

Newer style that have the internal adjustment screw.

How are you folks limiting the maximum vacuum advance ?

I am trying to decide if I want to go with a re curved Duraspark unit, (module type un determined at this point)

But the last unit I had in a running engine would pull almost 20 degrees at the balancer when given max vacuum from a mityvac.

I realize that some of that number was from the rpm increase caused by applying vacuum and some of that has to be mechanical as well.

If I were to use a duraspark I would like to tailor some type of mechanical hard stop for the vacuum advance as well as being able to adjust the vacuum setting via the internal adjustment.

I know some of you have don't this, just need some idea"s

Was thinking of a properly plased screw thru the plate with possibly a little cam that one could turn to limit the max amount.

Guessing that I would want 4-6 degrees, I think I could figure where to place the screw to get that based on how far the arm moves

minus whatever mechanical advance would occur as a result of that increase.

anyone want to throw any idea's out there ?

I hate to say it BUT the gm boys can just look up a part number or get an aftermarket unit that limits maximum advance with a turn of a screw.

                                  Boilermaster

 
I drilled and tapped the arm and installed a set screw with lock tite which limited the movement of the arm. I determined where to put the set screw by using a hand held vacuum pump and a timing light.

 
If you had not read very recent posts on distributors, curving and setting up vacuum advance canister, most if not all your questions are answered there.

If you do not happen to have an adjustable canister, you can buy one quite cheaply from RockAuto. I bought the Standard Motor Products VC31 for my 315C, which is adjustable with an internal screw. I think this is a typical canister for most Ford Motorcraft Dizzy's, but check your application.

They come with about 10* of advance set in. By turning the hex head screw counter-clockwise, you can dial in what you need. Mine is set to about 3-4* which seems fine for my engine and curve.

TommyK's idea would be a good fix if by chance your canister's mounting screws are seized and not removable without a lot of work.

Hope that helps,

Geoff.

 
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If you had not read very recent posts on distributors, curving and setting up vacuum advance canister, most if not all your questions are answered there.

If you do not happen to have an adjustable canister, you can buy one quite cheaply from RockAuto. I bought the Standard Motor Products VC31 for my 315C, which is adjustable with an internal screw. I think this is a typical canister for most Ford Motorcraft Dizzy's, but check your application.

They come with about 10* of advance set in. By turning the hex head screw counter-clockwise, you can dial in what you need. Mine is set to about 3-4* which seems fine for my engine and curve.

TommyK's idea would be a good fix if by chance your canister's mounting screws are seized and not removable without a lot of work.

Hope that helps,

Geoff.

Stanglover,

I was under the impression that the Standard motor products VC-31 has an internal adjustment for the AMOUNT OF VACUUM NEEDED TO START THE VACUUM ADVANCE moving and NOT the TOTAL amount of vacuum advance.

Can anyone confirm or disprove this ?

I do have an adjustable aftermarket vacuum advance unit (manufacturer unknown) that is known to be good and the adjustment has NO effect on the total amount the unit will pull, just the amount needed to get the unit to start.

please enlighten me

                             Boilermaster
 
The only unit that has a total adjustment is the Crane unit for GM points or HEI. The total must be limited by some mechanical means as TommyK stated. The adjustable units for Fords only adjusts the starting point and rate of vacuum advance. Chuck

 
Thanks C9ZX,

was pretty sure, but confirmation never hurts.

                        Boilermaster

 
If you had not read very recent posts on distributors, curving and setting up vacuum advance canister, most if not all your questions are answered there.

If you do not happen to have an adjustable canister, you can buy one quite cheaply from RockAuto. I bought the Standard Motor Products VC31 for my 315C, which is adjustable with an internal screw. I think this is a typical canister for most Ford Motorcraft Dizzy's, but check your application.

They come with about 10* of advance set in. By turning the hex head screw counter-clockwise, you can dial in what you need. Mine is set to about 3-4* which seems fine for my engine and curve.

TommyK's idea would be a good fix if by chance your canister's mounting screws are seized and not removable without a lot of work.

Hope that helps,

Geoff.

Stanglover,

I was under the impression that the Standard motor products VC-31 has an internal adjustment for the AMOUNT OF VACUUM NEEDED TO START THE VACUUM ADVANCE moving and NOT the TOTAL amount of vacuum advance.

Can anyone confirm or disprove this ?

I do have an adjustable aftermarket vacuum advance unit (manufacturer unknown) that is known to be good and the adjustment has NO effect on the total amount the unit will pull, just the amount needed to get the unit to start.

please enlighten me

                             Boilermaster
  May be, maybe not. While I cannot show it actually moving unless I take the distributor out and put it on a machine, I can see on my timing light exactly how much advance is being added at 3000rpm, timed port ( cruising speed rpms). The internal screw in mine is turned out 13 turns anti-clockwise from the all-in start position, to give me about 3-4*, not the 3.5 turns (10*)indicated on the instruction list provided. What I can see is how much the arm moves with vacuum applied with a pump and it's not much till it hits the end point or stop. It can't go any further no matter how much more vacuum is applied, but not doing that as it will damage the diaphragm.

So as I'm always keen to learn and improve my knowledge, I will be looking further into this as your information has me wondering somewhat and means I may need to change my set-up and re-evaluate prior information I was given.

Thanks for your comment,

Geoff.

 
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Boilermaster, do I feel like the village idiot or what!! Sorry if my previous post is/was misleading.

I got really curious about what I actually did, so I started over. I have 2 vacuum canisters. 1 was a non-adjustable on a Chinese POS distributor I got just for set-up, not to be run. The other was the Standard Motor Products VC31 which IS adjustable. As I don't need the retard feature, I went with a single diaphragm, hence the VC31. Somehow I got the two mixed up and installed the Chinese thing. No wonder I thought I had turned the screw out so far, there wasn't one!! Besides, it leaks anyway so it's NFG.

Anyway just now, I set the smp canister up with a dial gauge to measure the amount of travel on the arm using a vacuum pump. With the adjusting screw turned in all the way, clockwise, I measured .230" total travel @ 18-20"hg. With the screw turned out, counter clockwise, 3.5 turns according to the set-up sheet for the closest distributor listed for a 351C 4V, the total travel was .120". The 3.5 turns is a ball park figure as I do not have an exact reference, but it will be a start point.

What this DOES prove is that the screw does indeed limit the travel and once set at the desired advance with the timing light. you're done.

Sorry for any confusion guys,

Geoff.

 
I have a Crane vacuum advance that can be adjusted all the way down to zero advance. I took some cell phone pictures, but they didn't come out very good, I'll try to get some pictures tomorrow that shows the movement on the arm and the inches of vacuum.

 
So, time to eat MORE crow!!

I just took that Chinese canister apart to see for myself exactly how, what and why.

This one was adjustable, but I was using a ball end 2.5mm hex key and the socket part was not very deep, so it did not make solid contact. A square end key worked okay. What I found was that earlier comments on the workings of the vacuum canister were indeed more correct than I had surmised. The max travel is .230" and that is fixed by the slot length, minus the thickness of the peg. The diaphragm travel appears to be controlled by the spring pressure alone, increased or decreased by the number of turns of the adjuster screw. So it follows that if my engine is producing a max of 18" Hg, then that vacuum can only pull the diaphragm so far. The more the spring pressure, the less it will travel and therefore reducing the amount of advance. I could indeed turn the screw out 13 turns as I mentioned before, but the load on the spring was so great that the diaphragm hardly moved.

Pictures show the inner workings. I'll try to describe them as I go. #1 parts dissembled in order. #2 the screw adjuster as it fits into the outer housing. #3 is the adjuster screw and plate set at 3.5 turns. (At 13 turns it was all the way to the end.)

OK time for me to shut up on this. I learn every day and that's a good thing.

I probably should repost this in my own post, "which aftermarket distributor to buy......"

 
So, time to eat MORE crow!!

I just took that Chinese canister apart to see for myself exactly how, what and why.

This one was adjustable, but I was using a ball end 2.5mm hex key and the socket part was not very deep, so it did not make solid contact. A square end key worked okay. What I found was that earlier comments on the workings of the vacuum canister were indeed more correct than I had surmised. The max travel is .230" and that is fixed by the slot length, minus the thickness of the peg. The diaphragm travel appears to be controlled by the spring pressure alone, increased or decreased by the number of turns of the adjuster screw. So it follows that if my engine is producing a max of 18" Hg, then that vacuum can only pull the diaphragm so far. The more the spring pressure, the less it will travel and therefore reducing the amount of advance. I could indeed turn the screw out 13 turns as I mentioned before, but the load on the spring was so great that the diaphragm hardly moved.

Pictures show the inner workings. I'll try to describe them as I go. #1 parts dissembled in order. #2 the screw adjuster as it fits into the outer housing. #3 is the adjuster screw and plate set at 3.5 turns. (At 13 turns it was all the way to the end.)

OK time for me to shut up on this. I learn every day and that's a good thing.

I probably should repost this in my own post, "which aftermarket distributor to buy......"
I had never seen how they work on the inside, either. Thanks for taking the time and effort for doing this.

I connected a vacuum pump to my distributor with a Crane adjustable advance, to see what the effects of vacuum versus adjustment are.

I started with no adjustment, so maximum vacuum advance at lowest vacuum would be achieved. I took looked at the amount of advance at 5 inch increments required to achieve the maximum advance. I then did the same thing with the adjustable advance retarded (turned counterclockwise) by 2.5, 5, 7.5, and 10 turns.

The results are in the attached, and make more sense after you provided the above information.

Crane Vacuum Advance Effects of Adjustment.pdf

 

Attachments

  • Crane Vacuum Advance Effects of Adjustment.pdf
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Nice work Don C

Could you please run that set-up for all of us to see with the engine running and under all driving scenario's

Really all things considered , there is a LOT going on inside that little low tech distributor.

I am thinking there will be some gains to be had if one takes the time to tune the vacuum advance.

Looks like I am going to go with the crane unit and a duraspark distributor.

Still not sure on how I want to trigger it.

MSD, OEM duraspark 1, 2 or even considering a DUI HEI 4 pin.

Some will hide the GM module inside a duraspark case so it looks stochish.

                                  Boilermaster

 
My engine is partly torn down, and I will pull it and the transmission tomorrow. When it goes back in it'll have a roller cam, FiTech F.I. with a locked Duraspark distributor, and a 4R70W.

You are correct, a lot of variables in the distributor, when you look at what happens with the vacuum and mechanical advances, the possible vacuum adjustments and spring changes. And, it takes time to sort them out and then you have to consider what changes to timing does to carburetor tuning, and then how the spark plugs are handling the changes, and on and on. And then when we think we have it all straightened out we decide to change something else.

 
Last edited by a moderator:
So, time to eat MORE crow!!

I just took that Chinese canister apart to see for myself exactly how, what and why.

This one was adjustable, but I was using a ball end 2.5mm hex key and the socket part was not very deep, so it did not make solid contact. A square end key worked okay. What I found was that earlier comments on the workings of the vacuum canister were indeed more correct than I had surmised. The max travel is .230" and that is fixed by the slot length, minus the thickness of the peg. The diaphragm travel appears to be controlled by the spring pressure alone, increased or decreased by the number of turns of the adjuster screw. So it follows that if my engine is producing a max of 18" Hg, then that vacuum can only pull the diaphragm so far. The more the spring pressure, the less it will travel and therefore reducing the amount of advance. I could indeed turn the screw out 13 turns as I mentioned before, but the load on the spring was so great that the diaphragm hardly moved.

Pictures show the inner workings. I'll try to describe them as I go. #1 parts dissembled in order. #2 the screw adjuster as it fits into the outer housing. #3 is the adjuster screw and plate set at 3.5 turns. (At 13 turns it was all the way to the end.)

OK time for me to shut up on this. I learn every day and that's a good thing.

I probably should repost this in my own post, "which aftermarket distributor to buy......"
I had never seen how they work on the inside, either. Thanks for taking the time and effort for doing this.

I connected a vacuum pump to my distributor with a Crane adjustable advance, to see what the effects of vacuum versus adjustment are.

I started with no adjustment, so maximum vacuum advance at lowest vacuum would be achieved. I took looked at the amount of advance at 5 inch increments required to achieve the maximum advance. I then did the same thing with the adjustable advance retarded (turned counterclockwise) by 2.5, 5, 7.5, and 10 turns.

The results are in the attached, and make more sense after you provided the above information.
 Very interesting stuff there Don.

 For me, after a shaky start getting hold of what's going on with this timing / vacuum issue, I think I finally have a good basic understanding. I'll continue my experience in my own post so as not to take any more away from our friend Boilermaster.

I think his decision to go with the Duraspark and suitable "box" will serve him well. As for finding the best amount of vacuum advance, I found the "trial-and-error" method worked well. After all, it's a matter of adjusting one way or the other till you find the sweet spot. No listing or book has all the answers.

Geoff.

 
My engine is partly torn down, and I will pull it and the transmission tomorrow. When it goes back in it'll have a roller cam, FiTech F.I. with a locked Duraspark distributor, and a 4R70W.
Someone's got deeper pockets than do I!! Sounds like it'll a great motor.......... and you're NOT using an MSD thing!

 
No MSD for me.

I've seen how some folks stick an HEI module in a Duraspark can. Makes no sense to me. Duraspark is a good unit, MSD based their box on Duraspark, not HEI.

I've wanted to make these improvements for several years, and have researched all of the F.I. systems that have come and gone, glad I waited, FiTech fits what I was looking for (both cost and functionality). With 3.73 gears I need that overdrive. Monster has their 4R70Ws on sale, so it's time I stopped thinking and started working.

 
Great info Don, my setup likes 6 turns out on the Duraspark distributor which was achieved by trial and error in my case. I will break out the timing light soon to see what my total mechanical timing + vacuum advance ended up at under max vacuum situations. I am running 16 degrees base timing with 20 degrees of mechanical advance all in at 2800 rpm so I am curious to see what it ended up at with the vacuum advance added.

 
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