Jumping Timing Mark

SmokinBBC

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Oak Ridge, NC
Been discussed a million times over the years.

This is really starting to bug me. After I replaced the failed bronze distributor gear, I was getting the good ole wandering timing. So I pulled the dist and checked the end play. .025 on a dui hei aluminum body. Ok, so I shim it down to about .014. That should do the trick...nope.

Maybe it's the light. So I time it off the #6 plug.....dead steady. I move the clamp to all of the other wires....all dead steady on the balancer. So I figure bad wire. I swap the #1 and #3 wire. Still twitchty when timing on 1. 3 is dead steady. (Jump back and forth as much as 4 degrees every couple or so rotations) So it's not the wire.

Pull the cap and rotor. Maybe the #1 terminal is broken? Tested for continuity, tracking, etc. Cap looks to be perfect. The rotor looks to be in good shape with the exception of some reddish dust on the bottom side.

Wiped it down and put it all back together.........same crap.

What do you think? Cap/rotor out of phase? Cap/Rotor fried? When I first got it 3k or so miles ago, timing was steady on #1.


I was doing all this at roughly 675rpm with the vacuum advance plugged. When I bump it up to 1500rpm, it smooths out for the most part.
 
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Out of phase is easy to check, but I doubt that is the answer. The dimple on the gear should align with the rotor point.
Some timing lights are a little too sensitive, and can pick up on induced currents by other cylinder wires.
I revert to Mike, or call DUI and ask them.
Let me think on this.
 
Out of phase is easy to check, but I doubt that is the answer. The dimple on the gear should align with the rotor point.
Some timing lights are a little too sensitive, and can pick up on induced currents by other cylinder wires.
I revert to Mike, or call DUI and ask them.
Let me think on this.

Dimple on the gear is lined up with the rotor tip. I seperated the 1 and 3 wires quite a bit and had the clamp on at various points on the wire. What gets me is that the timing marks were all dead steady on the other wires.
 
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I would swear that my TPI FI can never did that when new, in full operation, but NOW, the timing jitters all over hell, but when I pull the tan/blk wire plug to like set timing? the jitter stops, it's fine, steady as a rock....

so go figger, it's been really weird that way....every once in a while the thing will go to a rough idle, but run fine at speed....

:surrender:
 
Just spent awhile on the line with DUI. We basically went through everything I checked. He didn't have any clear ideas. He offered to put it on their dist machine and give it a thorough checking which I may do if I can't figure this out myself.

A couple of "stretch" things:

Replace spark plug. he said it was a stretch that the plug could cause the problem.

Cap terminal problem. Basically, since it is only #1 with the problem, there could be a problem with that terminal. I might relocate #1 on the cap to see if the issue moves with the terminal.

In short, he couldn't think of anything else or any technical issues with the dist that might cause this problem..nor has he seen it before.




Just thought of something else. Anyone know the distance between the cap terminal and the rotor tip? Is there about a 1/16 or so gap that the spark has to jump? Maybe the distance between the rotor tip and terminal is too wide for the spark to consistently jump? What if I bent the blade on the terminal to close the gap a little?
 
I would change the cap, rotor, and plug before trying anything else. Worst outcome, spare cap and rotor.
 
The magnetic pickup coil will cause that

Misfires exactly every 8th impulse? I kinda doubt that.

I've had a hei do it. was on a crossfire car, would cause hit and miss sparks on several cylinder and other times run fine. felt like a 6 banger sometimes but one thing it did that you haven't mentioned is real bad fuel odor.

Have you checked the offending plug and made sure the electrode and insulator havent separated?
 
I would change the cap, rotor, and plug before trying anything else. Worst outcome, spare cap and rotor.

Thought the same exact thing. Just ordered a cap/rotor. For 30 bux, it'll be fixed or I'll have a spare.

Plug is going in tonight.
 
The magnetic pickup coil will cause that

Misfires exactly every 8th impulse? I kinda doubt that.

I've had a hei do it. was on a crossfire car, would cause hit and miss sparks on several cylinder and other times run fine. felt like a 6 banger sometimes but one thing it did that you haven't mentioned is real bad fuel odor.

Have you checked the offending plug and made sure the electrode and insulator havent separated?

I checked all the plugs a couple of weeks ago. Didn't see any problems. #1 looked like all the others. But, I am going to check/replace it tonight.

No fuel odor whatsoever. No lean odor....no rich odor. Starts up at 11.9AFR (choked). Idles at 12.8 to 13.0 AFR. Cruises at 13.9 AFR.
 
New plug went in....no change. Plug looked ok....maybe a tad rich.

Cap/rotor went in. Pretty steady now.....and it idles a tad better.

:beer:Thanks for the suggestions guys!
 
New plug went in....no change. Plug looked ok....maybe a tad rich.

Cap/rotor went in. Pretty steady now.....and it idles a tad better.

:beer:Thanks for the suggestions guys!

So the cap and rotor went south, or the cap carbon tracked. Did you use the nylon screws for the rotor? DUI uses them to help with scatter.
 
New plug went in....no change. Plug looked ok....maybe a tad rich.

Cap/rotor went in. Pretty steady now.....and it idles a tad better.

:beer:Thanks for the suggestions guys!

So the cap and rotor went south, or the cap carbon tracked. Did you use the nylon screws for the rotor? DUI uses them to help with scatter.

Yep. I used the nylon screws/washers. Silicone grease along with a rubber cushion for under the coil also came with the kit.
 
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