Timing mark walking

[Northy]

Hi there. I’ve got an 83SC (CIS Euro spec 204bhp) and I’ve noticed the timing mark will occasionally walk maybe 3-5 degrees more advance and then return to normal. There is no clear pattern, it just does it every now and then, like a few times over 10 seconds. The car idles well and sounds OK, the odd fluctuation, but it seems weird it’s not holding a stable 5 BTDC. If it was a misfire, I would have expected it to retard rather than advance?

I’m running CDI+, with the advance locked and controlled by the CDI+ The vacuum advance all works correctly, no radial play in distributor.

I’m still working through a slight missing or pulsing when off throttle and cruising through a village etc, I’m curious if it’s related to this and it’s a low level misfire or something (it’s not the bucking I had before, that’s been sorted with an airbox replacement).

Anyone got any suggestions why this might happen?

[neilbardsley]

How do you know the timing mark is moving? Are using the advance feature on a timing light? I gave up with my cheaper lights because I wasn't convinced the algorithm for estimating the advance was clever enough so only measure at zero adv on the pulley. Are you certain the rpm is completely stable during your test? Maybe test at a point in the advance v rpm is flat? Forgetting about the CDI unit for a bit jumping in timing can be caused by wear in the distributor drive gear and play in the distributor shaft. I would try to measure the dwell angle and see if stable across different rpm. If that is ok ask some about the CDI.

That said 3-4 degrees on an eye test doesn't seem too bad?

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[Northy]

Thanks Neil. I’m not using the advance on the timing light, just pointing it at the timing mark at idle. You can see the timing mark jump back left (advancing). On my car ROW SC 1982 - it’s 5 BTDC at idle and 25 at 4000 RPM, there are marks on the pulley for both, so it’s easy to see.

I’ve had a look at the dizzy and there is a bit of axial play perhaps 2-3mm max, but no radial play. Given the gear shape I wonder if some axial movement is enough to see this advance like that.

[PeterK]

Does your timing light also do revs ? If so, are the revs jumping a bit also ?

[Northy]

If they are, it’s ever so slightly. In other words, I can’t notice it.

[911hillclimber]

Not sure if I have ever seen a pulley timing mark 'stand still' at any engine speed, only in my limited experience.

[Ian Comerford]

Not sure if I have ever seen a pulley timing mark 'stand still' at any engine speed, only in my limited experience.

Agreed, but Lewis also checked my car with his timing light last week and it appeared a lot more stable than his. We are both running CDI+ but my distributor is not locked down. As a simple comparison there is a difference, whether it’s contributing to the engine miss is another point

[RobFrost]

I believe the Motronic uses the timing to fine tune the idle, alongside the idle control valve.