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Slop in the distributor drive?


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I found that my car seems to be changing timing for an unknown reason, one day it was advanced 12*(and corrected), a few days later it was 8* retarded. The MS tooth offset didn't seem to respond quite as I expected, but I'm not sure why.


I checked for slop with a dial indicator on the rotor, the probe was about 1 3/16 from the center. I measured about .045" worth of movement. This is the amount that it moved and stayed at after pushed on. I could move it more, but I suspect it was the plastic rotor that was flexing the most.

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Yea I watched your video on the MS section but didn't comment because I'm not sure if your problem is related to the slop in your distributor or not. I know I have some slop in my distributor too, if I take the cap off and wiggle the rotor it has some deflection, but really I don't think mine is that much different than yours.


I'm pretty sure that if I had enough slop to cause +/- 8-12 degrees timing change, that I would have blown my motor up by now. I mean at 21PSI of boost (the max I was running last year) MS was commanding 18 degrees of advance. If that timing had been able to change to 26-30 degrees of advance, at 12.5 AFR, I'm not sure that things would not have gone boom.


I'm kind of wondering if maybe it has something to do with the wasted spark system? I don't really know much about the wasted spark MS setups, but I would assume that you are running EDIS-6? If so is there anything internal to EDIS-6 separate from MS that may cause a timing change?

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Is this a static or dynamic change?

Meaning when you check with a timing light is the timing jumping all over (dynamic) a given range, or does it stay relatively steady at +/- 2 degrees of indicated setpoint on one startup, but the next time you start it, it's now offset by some number, and still operating with the same +/-2 degrees (static shift).


The distributor slop can give you spark scatter of the stated 2 degrees plus/minus. Anything more and you've got gear wear that is excessive. If its a ZXT CAS drive you won't even get that ammount of scatter.


The static shift is a sneaky one that JeffP and I ran across while on the dyno at JWT. The distributor drive gear slips on the drive shaft and timing is changed. In JeffP's ZXT, it jumped 180-Out! Both Jim and Clark scratched their heads with a "never seen that happen before, you obviously didn't drive here like that!" We dropped the pump, retimed the engine and did another pass, checking the timing afterwards: 8 degree shift! Did another pass, yet another timing shift!

Jeff installed a 3mm hardened pin to a new ZXT shaft and hasn't liked back.


Far as we can tell, it happens at higher Roma when you either have a fuel delivery problem and the engine surges violently (like you are stomping on the accel causing accel chop accel chop engine speeds), or when you lift throttle at higher engine speeds (JohnC mentioned oil pump tangs being snapped off under similar conditions)


On Jeffs engine it was nothing over 5500rpms when it happened.


From what you say changing day to day, it may be a slipping drive gear to the distributor/CAS assembly. Once they come loose, they can do it again easier at a later date.

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That seems to be exactly what I'm seeing(static shift), I'm also fighting a bit of an ignition issue, so there has been a fair amount of high rpm surging, but I believe I've tracked that down to bad alternator wiring. The only thing that doesn't explain is why a net zero change in trigger offset moved timing 8*.


I'm not sure I understand which gear is the one potentially moving, the gear on the pump/dizzy driveshaft?

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On my racing L6 the two tangs in the oil pump drive snapped off. I replaced the pump at the track and continued the race weekend. At the shop I pulled the pump/distributor drive shaft out and fit a .010" thin strip of hard brass shim stock around the bottom of the drive shaft tang so there was a tight fit in between the drive tangs of the oil pump. Solved the problem for at least the nest 50 hours on the engine.


Seems that vibration, reversing loads, and some excessive clearance in between the pump drive and the shaft tangs cause the pump tangs to fail.

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  • 2 years later...

Hey guys.  Do you mind if I restart this thread?  


We have a 280z motor running a 280z distributor and electronic ignition.  We are also seeing the timing retard 15 degrees.  The distributor was rebuilt prior to installation, berings were replaced, advance curve was modified, it was designed to be run with a vacuum advance connected directly to the intake manifold.  We are also experiencing a static change.


I noticed that with the car off, I can move rotate the distributor shaft with my hand to expand the centrifugal advance but when I return the rotor it stops  pre-maturely, I can force it to go back to zero.


Are you suggesting that this slop is caused by the distributor drive shaft?  Who knew!


Over the weekend, I plan to remove the distributor and try to see if the gear has slop.  If not then I guess I should focus on the distributor.


Any comments are welcomed.



Edited by JCan
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We have a 280z motor running a 280z distributor and electronic ignition.  We are also seeing the timing retard 15 degrees.



 it was designed to be run with a vacuum advance connected directly to the intake manifold.  We are also experiencing a static change.


When does the timing retard?  Not really clear what you're describing.  


And there aren't really any design elements needed to connect directly to intake manifold.  Once you open the throttle on ported manifold vacuum, it's connected directly anyway.  Connecting to full time vacuum just gives high advance at idle.  Ported vacuum gives retarded, more stable, idle timing

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To answer your questions

  •  the timing varies, we set it at 15btdc and check it after a day and it reads 0 btdc as an example.
  • we use our vacuum advance more as a switch.  when throttle is full open it retards, at idle it is full advance.

To describe our problem a bit better.


okay.  When I pull off the distributor cap and turn the rotor to full advance (clockwise) (stretch the springs) and let go the rotor does not stop at zero, it stops somewhere before zero, For the sake of clarity, i will call this location 15 deg. if I rotate counter clockwise from this 15 degree mark it will move there, but it almost feels lkike the shaft is hitting something very slightly.  


this could be an issue in the distributor or with the gear, maybe the gear slop in the Z axis is allowing the gear to move, maybe the gear is not attached to the spindle?  or maybe I actually have a screw that is hampering the return of the advance.  We have not yet pulled the distributor offf to try to rotate the spindle to determine if  the problem is with the spindle or the distributor.  Once we do, I will comment here.


Thanks for your help...  Jim

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Pretty easy to pop the reluctor wheel off and remove the breaker plate.  Then you can watch the weights and springs as you rotate the advance mechanism.  Hold the bottom and twist the top.  Something's not right in there.  It should move smoothly, weights moving outward away from the shaft, and snap back to where the weights are in by the shaft.  It does that over and over when in use, sometimes divots get worn in to the mechanism pieces.  Things dry out over time also, along with rusting.


Edited by NewZed
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