The Stovebolt Site Forums The Shop Area Engine Shop Vacuum advance?

When you rev up the engine, it's supposed to move right? As far as I can tell, mine doesn't. My engine runs fine though. I turned it with the engine running and it works, makes the engine run better, but I'm just wondering if something could be bad here?

Yes, you should see the distributor move with the vacuum advance, but only with a slight throttle opening. More than about a quarter throttle will make the chamber retard again if you snap the thottle. Make sure the line is properly connected and not blocked. Also make sure the vacuum chamber holds vacuum. If you have the vacuum line connected to manifold vacuum, the chamber will be fully advanced all the time. You can get manifold vacuum even at the base of the carburetor, depending on the carburetor. If you have a vacuum gauge, the proper vacuum port will have low or zero vacuum at the normal idle throttle position, and the vacuum will rise sharply with a small throttle opening.

Harvester

No, I don't have a gauge. It's hooked up the way it was when I got it and it appears original. Carter YF carb. If it works when I turn it manually, the unit SHOULD be ok? I tried it again and thought I saw it move, but I wasn't sure, how much should it turn?

Wrong. It could have a bad diaphram in it. Unhook the line from the advance, start the engine & raise it above idle. You should feel vacuum a the line. Turn off the engine advance the dis. by hand & put your figer over the inlet hole. release the dist. The dist should only go back part way till you remove your finger. If it snaps right back the vacuum chamber is bad.

Two ways that advance timing (distributor turns counter-clockwise) automatically...

Centrifugal advance - there are weights and springs inside the dist. housing that turn it more with increasing rpm.

Vacuum advance - connected to the vacuum port of the carb. Turns the dist. more with increasing manifold vacuum.


The vacuum adv. turns the dist. at idle and cruising speed, when you have high vacuum.
With increasing rpm and lower vacuum, the centrifugal adv. takes over.

There are different distributors and vacuum advances over the years.
For best fuel economy on a mostly stock and street engine, get the matching set for your engine and year.
Check the shop manual for specs.


A vacuum gauge is a must tool to have. It can help diagnosing the engine for various symptoms...
http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm

If you t-ed this tool to a rubber hose connected to the vacuum advance and suck on it, you know if the vaccum advance has leaks, when it starts to work and when it has reached max. pull.

Ok, will try what Wrenchbender said, I'm fairly certain that the diaphram is bad since my theory was wrong.

Make sure that your vacuum advance mechanism works freely. Quite often the linkage is bent or misinstalled. You must remember that people have been monkeying around with this thing for over 50 years. If you can freely advance the distributer by pushing the plunger into the chamber check for a vacuum leak. The old fashioned way of doing this is taking the distributor out. You have to do this anyway to replace the diaphragm. Suck on it to produce a vacuum. Stick your tongue in front of the hole. If it holds vacuum, it's ok. If not it will leak off or not build vacuum. I've been doing it this way for 50 years and it works.

First of all, I'm not all that comfortable taking the distributor off, that messes up the timing if you don't put it back on right, doesn't it? Second of all, I have to let it soak with pb blaster since the line is rusted to it. Third of all, I can advance it by pushing in the plunger. If it works when I turn it manually, would that mean that everything is clear and working right except the advance?

Our Langdon's HEI has a vacuum canister and the dizzy does not move and this fools a lot of old timers. The original 235 dizzy for this truck did move through advance. We have set our advance at 10 degrees but we still get a bit of a ping so we are going to try 6 degrees of advance and see what happens. BTDC -CW -before top dead will give you a retard on your spark and ATDC-CCW- after top dead center will give you advance. These guys know what they are talking about so follow their steps and you should find what is going on with your dizzy. T/C Steve

If you are not prepared to remove the distributor, you would probably be better off taking your vehicle to a mechanic. A mechanic would have a vacuum pump to check the diaphragm. If the chamber is indeed bad, and there is a good chance that it is, the distributer will have have to be removed to install a new vacuum chamber. These chambers are obsolete, but they can be found. Make sure a chamber is available before you have anything done.

Well, Crenwelge, I have people close(across the street close) to me that can take it off and put it back on right, but if I don't get it fixed soon it won't hurt anything would it?

It will not hurt anything to run it like it is. However it will hurt your performance and fuel economy because your timing will be late at speeds above an idle. If you have someone close to help you, make sure that you have a vacuum chamber available so that if it is bad you can replace it when you have the distributer out because the distributer has to be out to replace the vacuum chamber and then of course it will have to be timed after you put the distributer in. There are many old GM inliners running around with a bad vacuum chamber, and yours may have been bad for years.

That's good to here, except for fuel economy, but I'm not going to be driving it except for around the yard anyway(right now, I have 3 years before I can legally drive it on the road). Surprisingly, it does run great at any point, whether at or above idle.

You won't notice any difference driving around the yard. Once you get out on the highway, I'm sure you will want to fix it.