which distributor

If you can only get one with a double (or get a double one cheap), don't worry about it. Just use the fitting that comes straight out from the vacuum canister. Put a cap on the one sticking out at a 90 degree angle. It will work exactly the same as a single unit.
 
Howdy All:

Use a single if you have a choice.

The tap between the cannister and the distributor on the dual vacuum distributors is for vacuum retard, as Daniel said. IF you do have to use a dual vacuum distributor, DO NOT CAP OFF the retard port. Capped off it can limit the function of the advance side if it is sealed off. Best to just leave it open and forget it.

Page62- I'm curious about your experiences with the retard port capped off?

Adios, David
 
CZLN6":2cys19ta said:
Page62- I'm curious about your experiences with the retard port capped off?

Adios, David

Actually, my experience with these is on a Mercedes 450SL. I was only suggesting a cap for aesthetic reasons -- it didn't affect operation on the SL. But if in this case it can, I gladly withdraw my suggestion...

:)
 
Howdy Back:

Page 62- I, basically, did the same thing, thinking capping it would keep dirt out and look more finished. I noticed something was wrong in my gas mileage. I got checking timing with a dial back timing light at steady cruise rpm. At first I thought I had a bad canister or a wrong one. For some reason I pulled the plastic cap and, abbra- cadabra! Problem solved. Got the good mileage again.

To be clear and exact, my experience was with a '68 point type distributor. When I set up my DS II I used a distributor with a vaccum advance side only.

Thanks for "the rest of the story".

Adios, David
 
Using a cap would, in theory, limit the amount the diaphragm travels and thusly limit advance.

But there are a lot of factors that could negate this. I would just leave it open as capping it does no good.
 
as far as aesthetics are concerned, you can also route a vacuum hose from the retard port of the vac canister to someplace hidden, like into the air cleaner or beneath it.

looks fairly stock, but won't limit advance travel.

On some dual canisters, the retard port is merely a piece of brass tubing pressed into the canister body. You could pull that port out with some pliers, a simple hole looks less disturbing than an uncapped port.
 
I'm wondering why these dual advance retard mechanisms are so vilified.

My question is born from ignorance, so perhaps someone can enlighten me.

Is their purpose not to retard the advance during deceleration in effort to raise economy, limiting a rich mix of gas getting sucked in to the carb under those conditions. Is this not a good thing?

Do they in any way hinder performance when set up correctly?
 
Well, I'm not sure what their function is. I would imagine a temperature activated vacuum switch was probably used to retard timing during warm up, leading to more rapid catalytic converter function.

Anyone?
 
Howdy:

Jackfish- The retard function was designed to be part of a multi-function system. It did not work independently. Heat, load, EGR, thermactic air pumps and others worked in sync, in a perfect world. In fact all of those efforts were a crutch that, in some cases, inhibited both economy and performance.

Modern fuel injection has eliminated the need for most of those 70s systems. You'd have to look hard for someone to remember and understand, much less maintain, those systems. So, for most of us, they're gone.

Adios, David
 
8) the dual vacuum can on california emission equipped vehicles was use to reduce emissions. it had a vacuum port that was triggered by a temperature switch. as i recall it was designed to reduce ignition timing when the engine warmed up to reduce emissions.

for those of you that are building turboed engines by the way, use the dual diaphram unit, and plug the vacuum line in after the turbo outlet. that way you get a boost retard function automatically. it really works nice to eliminate detonation under boost.
 
well all those answers were more than adaquate. my pig farming military truck driving brain hurts with all that new info :shock: thanks guys.....or maybe gals too
 
Been chewing on this a bit and came across this in an old Chiltons:

"The second side of the dual diaphragm is the side that has been added to help control engine exhaust emissions at idle and during deceleration."

"The inner side of the dual diaphragm is connected by a vacuum hose to the intake manifold. When the engine is idling or decelerating , intake manifold vacuum is high and carburetor vacuum is low. Under these conditions, intake manifold vacuum, applied to the inner side of the dual diaphragm, retards ignition timing to promote more complete combustion of the air fuel mixture in the engine combustion chamber."

It seems to me this a good thing, and if I had a dual diaphragm dizzy I would probably hook it up and use it as it was intended just to test it out. Especially for on and off the gas driving, or anytime you're coming off high speed to a coast, would it not help lean out the mix and improve gas mileage? We have many threads looking at ways to improve economy and this may be an effect option as well.
 
Howdy:

Jack fish- good point. I've never done that exact test and no longer have a distributor with a dual vacuum canister. If someone does have that system please give it a try and give a report on the results. Better mileage is a worthy goal!!

Adios, David
 
I am running the DSII w/ a MSD 6AL standalone. Works great.

As far as the dual advance thing, hooking the retard port up to boost would generally cause you get MORE advance under boost, as the pressure would push the diaphragm.

I am going to be re-installing my vacuum advance pretty soon here, and my thought is that, with a dual port canister, the boost applied to the normal advance port will retard timing.
 
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