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Discussion Starter · #1 · (Edited)
Rally Bob, Otto, Travis, et.al., I've got a question that we've been beating to death in the chat room for a couple of days now. I thought I was pretty knowledgeable on the advance systems but this one has me going, so bear with me, please. In my Clymer's for the various distributors for the 1.9 engines for vacuum advances it has a max advance of 25 to 37 degrees, mechanical advances of 25-37 degrees from 3200-3600 RPM, with maximum total advances of 25-55 at 2500 to 3600 RPM. I was under the impression that the max vacuum advance took place at idle RPM and as you applied throttle the vacuum would decrease and so the vacuum advance would start to go into retard mode. By the same token, the mechanical advance would start to advance at above idle and as engine RPM increased the mechanical advance would increase to the max given RPM. So that as RPM increased vacuum would retard and mechanical would increase. So how do you get a max of 55 degrees total advance, when the vacuum drops? Also, do any of you folks have a chart of a given distributor that would show the vacuum advance at inches of vacuum or RPM and the same for a mechanical advance. I'm thinking it should be, if on a line graph two lines, one that starts way up, the other way down and as RPM increases the lines would meet about half way and continue across the graph as a very large "X". I really need input here to put my mind at ease, even though it won't be applicable to my GT. :confused: TIA

One more thing, for you racer types, that have limited the advances in the distributors, how did you do it, and what type dissy did you use. I've seen the threads trying to figure this out and I'm still in the dark. A little bit of help, O.K., a lotta bit of help on this please. TIA. :D
 

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Mechanical advance is controlled entirely by a combination of weights and tension springs, as you said. As engine RPM increases, centrifugal force forces the weights outward against spring tension and the weights' cam surfaces push against distributor central shaft lobes advancing the rotor and points cam assembly. THAT part you understand correctly.

Vacuum advance is controlled by PORTED vacuum, not MANIFOLD vacuum however! Most Opel distributors have a single canister with both advance and retard ports. The retard port is connected to MANIFOLD vacuum and is active only at idle with its vacuum line is further restricted by a metered orifice. PORTED vacuum for the advance port (sourced above the throttle plates) is ZERO at idle.

As you open the throttle, this PORTED vacuum rises in proportion to the airflow through the carb (venturi action) as engine speed increases and completely overrides vacuum retard. So both vacuum advance and mechanical advance increase cumulatively as engine RPM increases, one controlled by centrifugal forces, the other by increased airflow through the carb acting on the vacuum advance canister arm.
 

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advance and retard

Ron, I think the confusion revolves around the fact that there are two different vacuum sources to the distributor, and that they are both different from each other in source and in purpose.

The first vacuum source is one located in the intake tract below the carb, in the manifold, that is used to retard the distributor timing. This one acts as you described, that is as the throttle is opened and manifold vacuum drops it has a weaker vacuum and affects the distributor less. Specificly, it is an emission type device and it is supposed to have a maximum affect on advance during off-ramp style decelleration. In that type of engine situation the throttle is closed but the engine is still at a relativly high RPM, so manifold vacuum is very high and this forced the timing to retard as much as possible to minimize emissions.

The other vacuum source is attached at one end to the carb and at the other end to the advance side of the distributor vacuum pot. Internally to the carb this vacuum line gets it's vacuum from a small port drilled just above the throttle plate in the barrel of the carb below the venturi. This means the more the trottle is opened, the more vacuum is felt on the port, and as a result vacuum actually rises as the throttle is opened, with the limitation that if the throttle is tossed wildly opened this vacuum won't get any higher than manifold vacuum. This means that on a tip-in type event, where just a little more throttle is applied, the vacuum will go up, but it will return to basicly the same value as before once engine speed reaches a stable point at the new higher rpm. When you relate this to the distributor, it translates to a slight advance in timing that coincides with the tip-in on the throttle, both of which work together to speed up the motor.

Where this all gets interesting is at steady-state cruising. With no throttle or load changes, the vacuum below the throttle plate and imediatly above it, below the venturi, is very close to the same thing. This means the overall advance of the system is governed by the mechanical advance at this point. So what falls out is a curve for timing that is based strictly on the mechanical advance of the distributor, but modified by the vacuum advance to go up or down from there.

In a racing environment, it would be vitally important to run as much timing as possible, without too much advance, for peak performance. With 25 degrees of not well controlled vacuum advance available to do pretty much whatever, you would be forced to either limit the mechanical advance to be safe and loose power, or run the risk of too much advance melting your motor down. This is probably the biggest reason for eliminating the vacuum advance entirely on a racing or high performance motor. On most applications some variation of the factory set-up might be best, but the closer you want to operate near the limits, the bore vacuum advance you're going to have to rein in to keep the whole ignition system safe and predictable.

Or I could be totaly off base here and Bob will be forced to explain it better, maybe with pictures. :p
 

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Discussion Starter · #4 ·
Otto, Stephen, thanx, that clears up a bunch for me. I was basing my thoughts on this from old school basics, and normally aspirated aircraft, which both use manifold vacuum and standard flyweights for the advances. Ported vacuum was not/is not used in those applications. Maybe it's time to go back to the books again. But then on the V-6, totally computer controlled, I don't have to even think about all this, it's done for me. :D
 

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Re: advance and retard

oldopelguy said:
In a racing environment, it would be vitally important to run as much timing as possible, without too much advance, for peak performance. With 25 degrees of not well controlled vacuum advance available to do pretty much whatever, you would be forced to either limit the mechanical advance to be safe and loose power, or run the risk of too much advance melting your motor down. This is probably the biggest reason for eliminating the vacuum advance entirely on a racing or high performance motor.
The biggest reason to eliminate the vacuum advance with a racing or performance engine is simply because there is very little vacuum present because of the tendency of performance cams to reduce idle vacuum. Therefore the vacuum advance is very unstable and has limited effects.

Bob
 

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I need help on my advance problem. My advance has one canister on the distributer facing the driver side and one canister facing the booster. the PO had both of the vacumn lines tied into the ported intakes on the manifold. I now have one line on the manifold port ( from the canister facing the driver side) and the other one tied into the weber carb 9 the one facing the booster. Is this the right configuration ?? :confused:
 

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Dual canister distributor vacuum ports

baronbors said:
I need help on my advance problem. My advance has one canister on the distributer facing the driver side and one canister facing the booster. the PO had both of the vacumn lines tied into the ported intakes on the manifold. I now have one line on the manifold port ( from the canister facing the driver side) and the other one tied into the weber carb 9 the one facing the booster. Is this the right configuration ?? :confused:
Simply put, here it is:
 

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hello.. i was tinkering around with my car.. cuz it has a loss of power.. well when you rev it it seems to have a late reaction. and well it spitts a sputters when you get on it.. .. and when i pulled the two vaccum lines from the advance unit.. it ran exactly the same.. and unit didnt seem like it was sucking or blowing any air when i put my tumb over it.. im not exactly sure what that thing is supposed to do.. but does that sound like its broke? and would cause my engine to do that? and by the way.. how hard are they to find and to replace? thanks..
 

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I have only one port at the cannister on the distributor.
What hose shoulde be connected....the one from the carb, or the one from the manifold?
And what to do with the one thats over. Just close it?
 

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Which car?

ZmokE said:
I have only one port at the cannister on the distributor.
What hose shoulde be connected....the one from the carb, or the one from the manifold?
And what to do with the one thats over. Just close it?
Which car are you talking about . . . for clarity, post pic of your distributor here. :yup:
 

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I have the same single port vacuum advance. It should be connected to the carb. The vacuum retard which is attached to the manifold is strictly an emmisions gadget. Plug the connection to the manifold.
 

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tekenaar said:
Which car are you talking about . . . for clarity, post pic of your distributor here. :yup:
Sorry...it's an '73 GT.
I soon will publish a picture...now it's about midnight here..
 

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You might also want to check in the distributor to see if the advance mechanism inside is stuck. I've run into this problem on a few Opels. Probably won't hurt to add a drop or two of oil in the center of the distributor shaft directly underneath the rotor.

HTH,
Harold
 

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Here's the picture.
I connect the hose from the port at the distributor cannister to the base of the (Weber) carb now (first it was connected to the manifold).
The enigine reacts better when i give full throttle at once.
Only thing i have to do now is to close the small port at the manifold intake. I have to wait till i get back to work where they have such small hoses.
 

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Vacuum Advance....

The single canister vacuum control unit is connected by a small bore pipe to a fitting on the carburettor that is ported into the venturi area. At high degrees of vacuum the unit advances the ignition. Under load at reduced vacuum the unit progressively reduces the advance as the vacuum decreases. The purpose is to "light the fire" in rarified fuel air mixtures which take longer to burn than full load, wide open throttle mixtures. This advance helps the engine to run economically under high vacuum conditions like idle and over-run.
Unfortunately the Weber carb does not have this special port to connect the small bore pipe to as it is a high performance carb - not an economy one like the original Solex. The only place to connect the small line to is under the carb which is not ideal.
 

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Corrigan ....

High Vacuum at idle and on the over run with the throttle plate closed - not under acceleration when vacuum is LOW! Maximum mixture is entering the cylinders under acceleration so the vacuum is low - at idle and over run the minimum mixture is entering the cylinders and vacuum is HIGH.
Ignition is advanced by the high vacuum at idle and on over run to ignite the thinner - low pressure - mixture in the cylinders because it takes longer to burn.

HTH
 

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Hi Jim,

I am sorry, but you lost me. Which vacuum are you talking about? The Manifold or the Ported one from the carb?

What do you mean by over run?

Can you edit the section that needs modified and post it?

Thanks,
 

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Your pictured disti

ZmokE said:
Here's the picture.
I connect the hose from the port at the distributor cannister to the base of the (Weber) carb now (first it was connected to the manifold).
The enigine reacts better when i give full throttle at once.
Only thing i have to do now is to close the small port at the manifold intake. I have to wait till i get back to work where they have such small hoses.
Your pictured distributor is a 1975 FI, vacuum retard only (emissions at idle) type and would be properly connected to MANIFOLD vacuum, as it is operational only during idle! :yup:
 

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To determine the "action" of the diaphragm, remove the distributor cap and pull a vacuum on the hose (you can see enough movement even using your mouth). If the plate is pulled counterclockwise, it is an advance mechanism. If it is pulled clockwise, it is a retard mechanism. '75 distributors have single action retard diaphragms, which are connected to the intake manifold. This is to reduce CO and hydrocarbon emissions at deceleration and idle (high vacuum conditions). Most other distributors have two diaphragms or dual acting diaphragms, with the retard side connected to the manifold and the advance side connected to a port on the carburetor. The advance port is a drilled hole located just above the throttle plate. At idle, there should be almost no vacuum signal present here. When the throttle is cracked open, the throttle plate uncovers this hole, and the combination of the vacuum now present at the advance port and the reduced vacuum at the manifold port allows the advance mechanism to overpower the retard mechanism.
 
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