Rookie Dual Advance Vacuum Question...

68BlueStraight6

Active member
I am just trying to be clear that I have the vacuum advance/retard hooked up correctly on my Dual advance distributor. I have a 68 non SCV car with the dual distributor. I have the port closest to the firewall running straight to the baseplate of the Carb(autolite 1100). I then have the other port closest to the distributor body running to the Vacuum Distributor Control Valve then to the manifold seeing constant vacuum. I don't want to fire this thing up without being sure if this is the correct routing and the directions and diagrams are vague to me.

Thanks again for all the help.
 
:unsure: that's the opposite of what it should be. The front Diafram port (P) goes to the Carburetor Port, the Back Diafram port (S) goes to the Vacuum Distribitor Control Valve. Good luck :nod:
 
Bubba, you have been a huge help lately. Thank you for all your help. I am so used to ECU operated cars that this whole old school mechanical has been a trip.

Thanks Again!
 
If you want a better idle & don't require emission testing do not hook any hose to the inside nipple. This retards the spark & will lower your idle & make the engine sluggish at partial throttle.
I would replace that advance with a single diaphragm vacuum advance.
 
Howdy Back Blue and all:

Lets start by clearing up definitions. The nipple closed to the distributor is the inner nipple, that's the retard function side. The nipple farthest from the distributor is the outer nipple. It is the vacuum advance and should be connected to a ported vacuum source about midway up the carburetor.

Are you intent on maintaining the vacuum retard function for some reason?

Adios, David
 
I have no need for the retard function. So what can I cap to get rid of it? I just run one line from the carb spacer to the outer(closest to the firewall) nipple? And cap the vacuum source at the intake manifold, the two ports on the vacuum distributor and the inner(closest to cap) nipple? This would then cause the distributor to never see vacuum at idle? I think I need a picture cause I am totally flubbing this up.

Thanks for all your patience folks!
 
Howdy Again:

The "P" nipple in the diagram is the advance side. That hose goes to a ported vacuum source on the carb. There is no vacuum at idle from a ported vacuum source to the distributor advance. That allows you to use more initial advance. About 10 to 12 degrees initial.

The other side is the retard "S" mechanism. For performance, economy and idle quality it should not be used. I've found that I got best all-around by leaving it open and un-plugged. I disconnect and plug all other taps that that hose leads too. Especially at the manifold vacuum source.

That's what I did and it worked great for me.

Adios, David
 
Good advice from CZLN6. I still would purchase a single chamber vacuum advance for your distributor.
What distributor are you running??? Depending on your application there are many choices for a vacuum advance for you. Adjustable & non adjustable. I may be able to be of further help for you on build.
 
Okay so according to the diagram and you guys, just run a line from the P nipple which is closest to the firewall to the carb spacer. Cap the intake manifold vacuum source off and leave the S nipple which is closet to the radiator open to vent.
 
68BlueStraight6":2b77jxue said:
Okay so according to the diagram and you guys, just run a line from the P nipple which is closest to the firewall to the carb spacer. Cap the intake manifold vacuum source off and leave the S nipple which is closet to the radiator open to vent.

No sorry but the P nipple is the one that is closest to the Radiator! See the below link showing the correct vacuum line routing. Good luck :nod:

https://bzerob2.files.wordpress.com/201 ... 68_200.gif
 
Ahhh. Okay. So only run a line from the nipple closest to the distributor cap/radiator to the base of the carb. Leaving the nipple closest to the firewall open and unconnected.
 
I am not sure if you got this yet or if I am just getting confused by your discription terms. I think the confusion between us is because the disribitor body can be turned in the engine block about 180 degrees so I can't see which way your vacuum advance unit is pointing right now in relation to the firewall or the radiator. So let's try this one more time by just using only the distributor body itself and as CZLN6 also posted above. The Vacuum advance unit has two ports the front port is the P nipple this is the one farthest away from the distributor cap hook that up to the carburetors ported vacuum source nipple as in my above vacuum diagram. The back port of the advance unit is right next to the distributor cap this is the S nipple so you don't need to hook that one up if you don't want too. I hope this makes it a little more clear for you and if that's how you already have hooked up then your Golden and Bobs your Uncle. Good luck :nod:
 
Okay! Yes a bit of confusion there. And yes, that is how I have it hooked up now. Yikes, I think that became way more drawn out then it needed to be. Thanks again for all your help.
 
Howdy Back Blue:

DO NOT hook the vacuum line to the Carb adaptor. That is a full manifold vacuum source. Your distributor advance system is designed for a ported vacuum source higher on the carb. The ported source is slightly above the venturi on the outside of the carb.

Set the initial spark advance at 10 to 15 degrees. The higher end will likely be better.

Adios, David
 
Ah, sorry, yes that is where I hooked it up to. The port coming off of the carb. I had a senior moment in thinking it came off the spacer. I have the the brass block attached to the intake manifold blocked off and the port closest to the distributor cap. Finally think I have the thing tuned in with the timing set around 14 degrees. Just need to pick up a tachometer so I can double check the RPM the motor is running at. Just trying to get rid of the stumble when starting off and the occasional drop in idle and dying off. This looks like it may be a step in the right direction. Thanks Again!!!!
 
If any 200 six is changed to a Dual-Advance (swing-weights and vacuum) from a Load-o-matic (vacuum only), then MANIFOLD vacuum is used to the advance side of the vacuum chamber on the distributor. I highly recommend a SINGLE vacuum distributor because the vacuum chamber with 2 connections is for emission purposes only. Plug the connection closes to the cap if you have one. We don't need or want that. With the distributor set to about 10 degrees, which is 20 degrees at the crank (the limiter is inside the distributor under the points plate and it probably has 2 positions), and the static timing at about 14 to 16 degrees, that gives us 34 - 36 degrees total advance and we can run regular ethanol gas safely. I recommend 89 octane to be extra safe. I am 73 years old and spent decades building my own distributors from scratch (ASE certified engine builder). Mallory, ACCEL and others build good distributors but for street use they are usually an unnecessary extra expense. With top-of-the-line parts (I use BWD SELECT) we get brass/copper contacts (not aluminum) and the best over-the-counter stuff we can replace anywhere we travel. If you keep the 1100 carb, block off the ported vacuum and plug the spark-control valve. With manifold vacuum you have better idle and some advance once the engine rebuilds vacuum at speed. It then advances timing as you let-off the throttle or ease off when slowing down, giving a better burn. Just remember, the 1100 one barrel with a spark-control valve sends ported vacuum to the load-o-matic distributor, advancing timing as vacuum increases because the port is ABOVE the throttle plate. Manifold vacuum is tapped BELOW that plate so swing-weights advance timing as rpm increases, then vacuum advances as you slow down. It ain't as difficult as it sounds. Drag racing distributors (no vacuum chamber) work great at the track where they seldom see lots of idling, but on the street DUAL-ADVANCE is wanted. I rebuilt both race and street engines and they require different thinking where timing is concerned. Hope this helps.
 
There are so many variables of engine combinations that each application may be different.
With a stock camshaft normally ported vacuum works better. With a non stock camshaft manifold vacuum works better.
The key thing to remember is do not use manifold vacuum to cover up a wrong centrifugal advance curve.
On my distributors I normally use a lot of initial advance & shorten the mechanical advance. The 250" engine will not tolerate as much initial as a 170-200 series engine.
I have been using a Sun Distributor machine for many years so i can see on the bench what i want & need for a certain combo.
 
Howdy Back Blue;

Here's a thought. Charlie says, "Cap the retard side" I say leave it open. Charlie say, "Use Manifold vacuum." I say use the Ported Vacuum source on your '68, non-SCV 1100. Why don't you experiment with both and see what you get.

When I capped the retard nipple thinking dirt or other bad things would get in there I didn't get as much vacuum advance at at part throttle. No difference at idle or cruise. On manifold vacuum source I got too much advance at idle and cruise requiring less initial advance, which hurt low end power.

For comparison purposes, I'm 73 also and been playing with carb and ignition combinations since 1974.

Anyway, you've got enough info to try a few things for your self and see what you get. Keep us posted on your progress.

Good luck.

Adios, David
 
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