Wildly Wandering Timing

[madboy]

Had my roadster up on a hoist on the week, finally tracked down my suspension clunk, front left alignment nut was a 3/4 turn loose...so much noise from so little, oh well.

Anyway, while i was at the workshop, I threw a timing light on just to check things over(i have a miss at idle), so with gnd&10 shorted it sits on 16 reasonably steady, it was wavering about 1/4 of a degree either side, not really a problem, but when I pulled the short... whoa! damn thing won't sit still, its bouncing around between 13 and 19, everywhere in between with spikes off the timing scale. I showed the mechanic at work, but hes an old diesil mechanic whos into ealy model Torana's, he just shrugged his shoulders and said, "hmm something not right" and walked away....

I have read that wandering timing is fairly normal, but not to this scale. To give some history, plugs and leads were changed 2k5 km ago, timing belt was changed recently and everything was very tight, no wear in the keyway or pully(long nose crank).

Is this even a problem? or a symptom of something else not working properly? Is there a plug in diagnostic that can be used to check for problems?

MB

[Matty]

Was the idle smooth? If so, this is totally normal.

[madboy]

Was the idle smooth? If so, this is totally normal.

my idle is reasonably smooth yes, the tacho sits on about 750 and wavers down and back up about 50rpm but I have a miss that seems to be pretty regular at warmed up idle.

[JBT]

....but I have a miss that seems to be pretty regular at warmed up idle.

That'll cause the spikes.

[madboy]

....but I have a miss that seems to be pretty regular at warmed up idle.

That'll cause the spikes.

cause or result?

[Steampunk]

Don't worry about it, it's normal to have the timing bounce 2-3 degrees +/- from what it is set to.
I can't find the link at the moment, but if you search one of the big US forums it will confirm this.

[JBT]

....but I have a miss that seems to be pretty regular at warmed up idle.

That'll cause the spikes.

cause or result?

If the engine is missing the timing light indication will vary (spike) and this will be particularly noticeable when the ECU is in play i.e. TEN - GND unbridged.

[madboy]

If the engine is missing the timing light indication will vary (spike) and this will be particularly noticeable when the ECU is in play i.e. TEN - GND unbridged.

ok, so i really need to track down what is causing the miss... any ideas, plugs and leads are only 2k-2.5k km's old...

[Juffa]

ok, so i really need to track down what is causing the miss... any ideas, plugs and leads are only 2k-2.5k km's old...

Your idle of 750 Rpm seems a little low. Shouldn't it be around 900 Rpm? How old is your O2 sensor?

J

[JBT]

Missing could also be caused by other ignition components such as a faulty ignitor/coil. All you can do most times is replace and hope, unless you can find a test facility or someone who can loan you a known working unit(s) to fault find.

[madboy]

ok, so i really need to track down what is causing the miss... any ideas, plugs and leads are only 2k-2.5k km's old...

Your idle of 750 Rpm seems a little low. Shouldn't it be around 900 Rpm? How old is your O2 sensor?

J

I replaced the 02 sensor about the same time as the plugs and leads, used the single wire bosch unit from repco.

Is there such a thing as a plug in disagnotic for the early NA's? Or does anyone know of somewhere on the gold coast that knows mx5's? I heard of one in the Labrador/Southport area, some guy who races them, but cannot remember the name.

[manga_blue]

Mine used to wobble about like that too. It turned out that the timing was perfectly consistent but the crankshaft pulley was loose, causing it to twist back and forth on the shaft. This meant that timing mark, which is on the rim of that pulley, was also wandering around, giving false timing readings.

[Matty]

what JBT said.

The timing is jumping around as an attempt to control the idle speed. This is much more controllable than doing it by fuel management alone.