Timing mark jumping around
Thread Starter
Instructor
Joined: Oct 2006
Posts: 181
Likes: 0
From: Des moines Iowa
Timing mark jumping around
I went to set my timing and i noticed the timing mark jumping around. It was moving a couple of degrees hard to pinpoint actual timing. What would cause his??
Burning Brakes
Joined: Oct 2004
Posts: 920
Likes: 1
From: Shelby Township Michigan
Before looking for a mechanical problem, make sure you have the clamp placed properly around the #1 plug and that it is not touching anything. Fire the gun at some spot other than the timing mark and make sure it is constant with the engine rpm.
Good luck,
Dino
Good luck,
Dino
Racer
Joined: Jul 2003
Posts: 455
Likes: 0
From: Buffalo New York
I would start with the timing light as mentioned above, if it is reading another wire it will jump around. After that, I would check spark plug gap, wires and then the distributor...
Good Luck!!!
Chris
Good Luck!!!
Chris
Drifting
Joined: May 2006
Posts: 1,425
Likes: 0
From: Seattle Wash.
What he said. You are experiencing "timing scatter" The higher the RPM's, the more your distributer if vibrating up and down. I bet you could grab the top of your distributor cap and feel movement up and down. Ask me how I know. Replace the bushings. Good luck!
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Tech Contributor
Joined: Aug 1999
Posts: 14,373
Likes: 6,371
From: At my Bar drinking and wrenching in Lafayette Colorado
There are a few things that will cause "timing scatter:"
So yank the distributor out and tighten it all up. This will produce rock solid timing.
- Excessive distributor shaft endplay. If there is excessive play in the shaft, the shaft will bounce up and down, advancing and retarding the timing. Shim the shaft to about .007" endplay using Moroso shims part number 26140.
- Slop in the breaker plate attach point. The distributor breaker plate is located in position with a hole for the vacuum advance control unit rod. This hole gets sloppy, and the plate bounces around. Remove the plate, lay it on a flat surface, and whack the hole with a hammer to tighten the fit yup and remove all slop.
- Bad distributor bushings. If the bushings have slop and the shaft has lateral movement, timing will be eratic.
So yank the distributor out and tighten it all up. This will produce rock solid timing.
Pro
Joined: Sep 2004
Posts: 595
Likes: 0
From: Boston MA
I would start with the timing light because it is the easiest and cheapest place to start.
If the problem continues….
What type of advance springs do you have installed?
By that I mean what is the stiffness, if the they are not very stiff and the idle is high, then you may be experiencing some advance during idle. (Some early high performance models were actually setup this way.)
What is your idle set to?
What type od distributor?
What type of springs?
Also, it could be the vacuum advance. It may not be fully engaged at idle, you may not have enough vacuum to keep it open. (if it is connected to manifold vacuum.) The easiest way to solve this potential issue is to pull the vacuum hose off and plug it at the carb. See if the issue stops.
What type of vacuum do you have your vacuum advance connected to?
What is vacuum reading at idle?
What is the vacuum unit rated at? (or give me the model number)
Thanks,
Joe
If the problem continues….
What type of advance springs do you have installed?
By that I mean what is the stiffness, if the they are not very stiff and the idle is high, then you may be experiencing some advance during idle. (Some early high performance models were actually setup this way.)
What is your idle set to?
What type od distributor?
What type of springs?
Also, it could be the vacuum advance. It may not be fully engaged at idle, you may not have enough vacuum to keep it open. (if it is connected to manifold vacuum.) The easiest way to solve this potential issue is to pull the vacuum hose off and plug it at the carb. See if the issue stops.
What type of vacuum do you have your vacuum advance connected to?
What is vacuum reading at idle?
What is the vacuum unit rated at? (or give me the model number)
Thanks,
Joe
Race Director
Joined: Apr 2003
Posts: 19,347
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From: Tampa, Florida
Tech Contributor
Joined: Aug 1999
Posts: 14,373
Likes: 6,371
From: At my Bar drinking and wrenching in Lafayette Colorado
Lars
Le Mans Master
Joined: Apr 2004
Posts: 7,106
Likes: 38
From: Newark DE
Kev
listen to Lars advice on your dizzy. As he says, excessive shaft play, play in the breaker plate, and/or bad bushings all will cause the problem you are having. At the same time, the erratic timing will cause you to never have the timing set correctly and this will also affect the timing and vacuum in your other post.
If you don't know how to rebuild your distributor, there is probably someone local that can do it for you. You might also check with Lars but while he rebuilds points distributors I'm not sure if he also offers his services on HEI units or not, I can't remember.
listen to Lars advice on your dizzy. As he says, excessive shaft play, play in the breaker plate, and/or bad bushings all will cause the problem you are having. At the same time, the erratic timing will cause you to never have the timing set correctly and this will also affect the timing and vacuum in your other post.
If you don't know how to rebuild your distributor, there is probably someone local that can do it for you. You might also check with Lars but while he rebuilds points distributors I'm not sure if he also offers his services on HEI units or not, I can't remember.