I was checking out my dial back timing light yesterday. With the car idling the timing was jumping around. If I got on the gas a little it would settle out and be steady. What could cause this? Could this be the cause of my off idle stumble? The vaccum advance is connected to a ported point. Any ideas.


Thanks for any help guys.

 

Vacuum leak, worn/loose timing chain, point plate loose/distributor shart loose...

 

also look at advance springs ,wts. and bushings right under rotor

 

Got new timing chain and gears. I will try it today with the vac to the wiper doors and such blocked off and see if that makes a difference. Thanks for the tips.

Anyone else.

 

if all looks good test timing light on another car

 

Vertical play in the dist. shaft will cause timing wander too.

 

Play in shaft, or....... what are you using? Points, HEI, pointless conversion??? My pointless conversion was causing erratics on my 71.

 

A couple of things will cause the symptoms you describe:

1. If you have "stock" amount of endplay in your distributor (vertical shaft movement), the shaft will bounce up and down a bit at low rpm. Since the distributor gear is cut at an angle, vertical movement of the shaft will cause the timing to change (timing retards as the shaft moves upwards). At higher rpm, the shaft is pushed all the way up against the shaft washers, so timing will stabilize. To deliminate the timing "scatter" at low rpm, you need to shim the endplay out of the distributor. Endplay should be about .005".

2. If you have a quick centrifugal advance curve with a soft set of springs, the centrifugal advance will start coming in right off idle. Since you're right at the beginning of the spring tension, low rpm can produce some erratic timing.

3. A very common problem is that the point plate (the plate that the points mount to) has a sloppy hole interface with the vacuum advance control rod. The vacuum advance control rod is the only thing that holds the point plate in its correct timing position. Of the hole is wallowed out, the plate will "rattle" back and forth as the points go from cam lobe to cam lobe. This is most noticable at low rpm. If you can move the plate slightly back and forth with your fingers, you have a problem - the plate should feel solid, with no looseness. To fix, remove the plate and whack the hole wth a hammer to tighten it up.

4. If you're running a PerTronix system, these are notorious for "timing scatter." Install a good set of points.

 

Lars would this contribute to an off idle hesitation?


[Modified by Eddie 70, 6:10 PM 1/5/2004]

 

Eddie -
The erratic timing by itself will probably not give you an off-idle stumble. However, if your timing is not set up right, and you are not runniing enough initial advance, you can certainly get a stumble.

If you have a bit of wear or age on your distributor, the advance stop bushing is most likely missing. It's also quite common for the vacuum advance control unit ("vacuum can") to be inoperative on an older system. These two things combined can produce a retarded condition at idle, resulting in the stumble. For best performance, total mechanical timing should be 36 degrees. With this at 36, intial timing should be in the mid teens. With vacuum hooked up, initial timing should be in the mid-to-high 20's. If you're not getting numbers in this basic range, you need to do a little distributor work.

I do have a distributor machine, and can do the setup for you. Contact me if you want help with it.