Engine Timing and Distributor Advance

[Lifted79CJ7]

OK, here is the only real part of my Jeep I don't really understand. I understand how important it is to set timing, but I don't understand (1)vacuum advance and how to check it. (2)Also, how can you tell if you need to replace your points? (3)Oh, and what are points? May seem like something simple, and frankly I am quite surprised I've never looked this up before! (Might expleain why my 304 ran like crap!!!)

(4)Also, what all should you be looking for when setting your timing? I have seen people pulling vacuum lines, etc while doing a complete tune up, but which lines were they pulling?

One last question. (5)What does the ignition control module actually due on my factory Motorcraft ignition system in my CJ?

I know this is gonna take a lot of typing, but if anyone has a few minutes to help explain some of these basics to me, it would be greatly appreciated!! :t:

:-| Jack

[TommyD]

I'm not a master on points but let me give a run down on how the whole system works.

Early distributors used an internal mechanical switch known as points. From 1978 thru ? Duraspark distributors were used which had an internal magnetic switch instead of points.

This Duraspark distributor has on the main shaft a magnet. On a 6 cylinder this magnet has 6 bumps and on an 8 cylinder it has 8 bumps.

Somewhat stationary inside the distributor is a switch (orange and puple wires going to it. This switch is actuated by a magnetic field being passed in front of it. So in other words when ever any of the bumps on the magnet pass by this switch it sends a signal to the ignition module telling it to fire the coil.

Now just think of points as an older way of doing the same thing. Bumps on the distributor shaft (instead of a magnet) actuating a switch (points). Now the down fall with points is that they are mechanical and after a whole lot of opening and closing with voltage on them they get crusty from the voltage arcing and the spring used to return the switch to the open position looses tension. You also need to be concerned with the gap in the points, which affects how long it takes the switch to close. You also have to be concerned with moisture as this may make the swith activate sooner or later than it's suppose to.

The only purpose of either one of these setups is to tell the coil when to send spark back to your distributor, so the distributor fires when its rotor is right on one of the terminals on the distributor cap. If you didn't have this system, the coil would be on all the time and you would have high voltage inside the distributor cap arcing to any terminal or terminals it's close to and possibly firing more than one cylinder at a time.

Now onto timing. As you know, timing is done by rotating the distributor. What you are really doing is moving the switch (points) or actuator so that it hits the bumps earlier or later on the distibutor shaft and sending the spark to the plug at the best time during the stroke of the engine.

Vacuum advance is used for when the enigine is at higher RPM. At higher RPM your engine runs more efficiently by getting spark ealier during the stroke. Because of this you need a way to move the points or magnetic switch inside the distributor so they hit the bumps sooner than they would at lower RPM

That little canister on your distributor is a diaphram that moves when it has vaccum applied to it. The higher the RPM the more vaccum pressure the more movement of the diaphram. Now your points or magnetic switch (depending if you have old or new) is all riding on a braket inside the distributor.

When vacuum is applied to the diaphram it actually rotates this bracket inside the distributor and advances the points or magnetic switch so they hit the bumps sooner and your engine gets the spark sooner in the stroke.

When you time your engine, you want to remove the hose going to the diaphram on the distributor and plug it, so you don't have a vacuum leak back to your carburator.

What you are really doing is making sure that no matter what RPM your engine is at, you can time your engine without having to worry about the points or magnetic switch being advanced by the vacuum and you get a true base timing.

I hope this helps.

[Lifted79CJ7]

Ahhhh, gotcha. I just always assumed that the coil was always on all the time and that it didn't arc all over the place. So that completely explains the vacuum advance, etc as well. Thanks for explaining that! I was always timing mine with the vacuum attached. :oops: Maybe this 401 will run a little better than my 304 did!!
Jack