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13,432 Posts
Your asking some good questions. I just hope TR doesn't fall of his chair laughing at my explination... He's a pro... me, I'm a backyard mechanic... so here goes...

Timing is a combination of two things:

1. A Starting point (static), and...
2. A Moving Point (dynamic).

So, let's reason together on this one...

First, anything is only as good as it's foundation. In timing an engine, that's the Static Timing. It is this point from which all other things flow on a non computer controlled engine. Click on the Timing gun below and see how to time an engine to the nats eyebrow.

If you notice, you're timing the engine with the engine OFF. This ensures that the foundation is good.

So now start the engine and you should be running just fine...unless you've tweeked and played a bit trying to mask a problem. This is because the engine is a slave. All it does is make that crankshaft go round and round. Same ol' after same ol stuff revolution after revolution.

So what can go wrong? Yep, it's that Moving Point thing... you've got to nail jello to the wall!

Well using your example, by putting the engine in Park or Drive you're placing a load on the engine, or you're removing it. The manifold vacuum is being used to monitor the engine load. As the load increses the manifold vacuum increases unless more gas is given, raising the RPM thus decreasing the manifold vacuum.

So, as you can see, in park or in drive it don't make a bit of difference. Vacuum is Vacuum. What you're concerned with is how the enigne reacts to the load. This reaction is a combination of two items:

1. Base RPM.
2. Vacuum advance.

If the Base RPM is at say, 800 rpm and the engine is put into Drive, the RPM drops at least 50 RPM. The manifold vacuum increases, or lopes. Then you raise the idle to 850 RPM and put the transmission in Park. The engine races up to 1000 RPM when the load is suddenly removed.

So where's the problem? Well, it's got to be in the advance system. This is made up of the:

Timing Gears.
Timing Chain.
Vacuum Advance.

So how do you know you've got a good timing gear / chain set? Well that's easy, go back the the foundation rule.. and check it statically.

A Jeep does not do well if there's a lot of slack in the timing gear/chain set. So try the following:

1. Remove the Distributor cap.
2. Observe the poistion of the rotor.
3. Put a socket and ratchet on the crankshaft pully.
4. Get a friend.
5. Have the friend look at the rotor.
6. Use the ratchet to manually rotate the engine.
7. Note the starting position of the ratchet handle.
8. Slowley rotate.
9. Have the frined yell when he starts to see rotor movement.
10. Stop the rotation!.
11. Observe the handle of the ratchet.
12. How many degrees did the handle move?
13. If it's more than 5 degrees, replace the timing chain/gears and retest.

If the engine still lopes then the problem is in the Vacuum Advance of the distributor.

Now, see how easy this was? An we didn't really get into the Park/Drive thing.. cuz' it's really not needed. Just keep about a 75 to 100 RPM difference between Park and Drive. Clear as mud!


13,432 Posts
Nope.. if the trannie was causing too much drag...then you'd be hearing lots or real bad, ugly, noises from under the floorboard and the smoke would be driving you out of the cab.

I'd suspect that you've got a setting on the Carb wrong... like the curb idle setting. Crank down the Curb Idle and see if you can still keep the engine running. I worry when there's more that 100 RPM difference between drive and park. You're no doubt hearing things slam into gear with that autobox?

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