The Stovebolt Site Forums The Shop Area Engine Shop vacuum advance or vacuum retarded???

I have a 55 1st Chevy truck with a 55 235 that has a vacuum advance that will not work. The question is does this vacuum pull to advance or release to advance? I have been told both ways!! When this engine is started the vacuum advance immediately pulls all the way in and never releases. You can pull it out with your hand and it goes right back in. Someone said that it should pull in like that then release during acceleration. Someone else said it should be at rest when idling then pull in during acceleration. Seems to me it should be at rest until acceleration then advance the time by pulling in. I have taken it off and you can push it in then hold your finger over the hole and it will not release until you move your finger which makes me think it is good but it will not move the distributor at all, I think because it stays pulled all the way in all the time. So, is it me or the vacuum advance that is retarded?? How is this thing supposed to work?? thanks!!! slowgoer

Basically a working VA will retard the timing with decrease of manifold pressure.

You can check your VA with a Vacuum gauge and suck on it to determine the starting and max. range in Hgs.

Distributors and VAs look similar or alike for these engines over the years.
However there are quiet some differences.
Check you engine year's manual to find the correct part numbers.
There are also different numbers for cars and trucks.

Base timing also plays a role to get it all tuned properly.

If your engine has been upgraded from stock, you may have to compensate or modify the distributors springs and weights and/or get other VAs with a different range.

Another thing to think about is if you are using a PCV system with a valve that drops too much Hg.
If you go by todays PCV valve suggestions for this engine, that's the case.
After experimenting I found the perfect valve with the least drop. You can search the archive for a post I did last year, which has the part number I ended up with.

Of course always check for vacuum leaks.

It advances the timing with the application of high vacuum, such as when idling or light accelleration. It should release partially at high rpm operation, the mechanical advance is in operation at that point. Seems to ma that if it's pulling in and holding under vacuum, it's ok. You should try checking it on the engine by having an assistant operate the throttle as you watch the action of the advance. It should pull in imediatly when the engine starts, then release as the throttle opens up.

slowgoer,

I might be as 'retarded' as you. I will give my version and let the experts set us both straight.

If you can draw a vacuum, with your mouth or a pump on the vacuum advance unit, and it will remain retracted until you release the port, the unit is good. You did this and proved yours to be fine.

Engine vacuum is highest at idle. At idle, your unit should have pulled the distributor counterclockwise and held it there.

If the distributors normal rotation is clockwise, then a vacuum retracted unit will hold the distributor in a advanced state until vacuum drops, and the internal spring shoves the distributor back towards it static position.

When you loose vacuum, it usually means an increase in engine RPM, which will bring the centrifugal advance into play.

The system will never go into a 'retarded' setting. I think you have only initial advance, which you set with a light, then you have vacuum and mechanical advance. You can never get past initial advance, so in theory, cannot get to a retarded state.

Your vacuum advance unit has to move the distributor pretty easily. You should see it do this. If you are using the port on the carburetor intended for this function, as soon as you crack the throttle, that distributor should move.

The mechanism must not be bound. The internal spring inside the vacuum advance unit is quite stout, but things must be free.

Some types of distributors only move the plate inside the distributor housing, but I don't think any of the old GM's were like this.

I might have to eat some humble pie, as this may not be the way it actually works.

Stuart

Thanks for all the info! Apparently I was wrong!!!!!!!!!!!!!!!!!!. I will check again to see if it releases with throttle although before I didn't notice any movement at all. As for as binding, it seems to be free to move and the spring is quite strong. If it is not releasing what should I look for. Someone mentioned internal or mechanical parts in distributor I think. thanks again! slowgoer

Just to add a little info. the reason it's retarded at rest is to make starting a little easier. Once started the VA advances the timing to "normal" position.

When the engine is timed with a timing light, at idle, with the vacuum advance blocked, I think it is called 'initial advance'. If you add this value (in degrees) to whatever value you achieve with centrifugal and vacuum advance, you get 'total advance.

I don't think 'initial advance' will ever be ATDC, or retarded. Every timing value I have ever seen is BTDC.

So, technically, timing will never be retarded. It might be 'less advanced' but never ATDC.

I think this is correct!


Stuart

When you accelerate the engine the advance should drop back. If it doesn,t the hose or tube could be partialy stopped up. Some engines run it off of full manifold vacuum. That would have vacuum advance at idle. Some engines run ported vacuum. The line comes out the side of the carb & will have vacuum advance only at light throttle. Set the initial timing with the vacuum disconnected.

slowgoer,

To check for any binding, merely grab the distributor by the cap (engine off) and rotate it CCW...it should rotate freely, and should pop right back.

There should be no internal concerns. The vacuum advance diaphragm rotates the entire distributor.

Stuart

I think I made a 'erroneous' statement in my previous post. It appears that the ported vacuum is "above" the throttle plate, so at idle, it See's little vacuum. As soon as the throttle starts to open, vacuum is sensed, and the advance unit advances timing accordingly.

Watching the distributor while working the throttle by hand would give you a idea of how the thing operated.

Stuart

I think your right. My 58 advances only as the throttle plates open. No or very little vacuum at the dizzy at idle. Only when the throttle plate opens does the vacuum begin to work with the advance.

After a point though the mechanical advance takes over when wide open throttle exists and the vacuum drops to nothing. Especially on long hills or under heavy load.

This vacuum line is coming off the carb port so are you saying it should have very little vacuum at idle and then advance the timing at acceleration? This one pulls hard at idle and it only moves very, very little to retard when you rev the engine. By the way, how do the old guys time these engines just by listening to them. What are they listening for and how close to the correct time can they get? slowgoer

slowgoer,

Yes, that is how it is suppose to work.

It sounds like the vacuum line is coming off the wrong spot in the system. A Rochester carburetor has its ported vacuum tap right next to the idle adjustment screw.

I have heard you can time this engine by ear...rotate the distributor for maximum idle speed , than roll it back (CCW) a "hair". I have no clue if the actually works.

I think a timing light would be much more accurate..is 60 a 'old guy'.

Stuart

One other thing, what I am trying to stop is the engine bogging when you try to accelerate. Everyone says the timing is off. It has new plugs, points, condenser, rotor, cap and wires. The carb has been rebuilt and only has one adjustment screw on it, I think. Normally, how many turns out should it be from the full in position? Could there be a problem with the carb rebuild job that I should look for? thanks a ton

Stuart; I don't know if 60 is old or not but I am not as far from it as I would like to be. I was referring to a man here who is about 79 I think. He drove these old engines for years in his construction business way back in the day and can do just about anything with these things just by listening. I haven't called him yet but it may come to that.

slowgoer,

A standard idle adjustment on these carburetors is 1 1/2 turn out. I don't think this would be a concern above a idle.

Your problem sounds almost like a power valve issue.

Typically, there is a phenolic spacer under the carburetor. The gasket between the spacer and the carburetor base has small slots that have to line up to a port in the carburetor base. This allows the carburetor to sense vacuum and signal the power valve to operate.

If you DO have this port exposed, and the setup isn't working, this along with the malfunctioning vacuum port leads one to think the rebuild might have been botched!

Could this be the case? Was the rebuilder a competent carburetor man or experienced mechanic.

The timing could be a problem too, but this would be secondary to getting the vacuum advance to operate as it should. Keep looking!

Stuart

Thanks again for the info!! I will first check the spacer under the carb then if need be I'll go further. slowgoer

sounds like you have vacuun advance connected to constant vacuum...it should be connected to a port that omly pulls vacuum when accellerating...i've seen alot of them hooked to the wrong port.