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Hi All, I was setting up the ignition timing on my friends 1995 1800 MX5 with ITB's and MS2 and when we set it to fixed timing of 10 degrees and checked the timing on the engine with a timing light, the timing seemed to be jumping back and forth, so say like between 12-8* instead of just sticking on 10* like I would've expected and as mine does! Once we increased the RPM to about 2000+ it seemed to stabilise and had a lot less movement and was at about 10*.

So I was wondering if it could be something like the cam timing or maybe an issue with the timing belt or something like that causing the issue?!

Also just wondered if this is common on MK1 1800's or if anyone has seen this before and what could be causing it?

Thanks very much
Rob


P.S. Car is running Megasquirt but I would assume it would do the same thing on the stock ecu as it was set to fixed timing in TS so the ignition timing shouldn't have been moving at all other than spark latency at higher RPM's.
 

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A not tight enough timing belt could cause this jitter.

The downside to the CAS is that its accuracy is entirely dependant on the amount of slop on the timing gear. Imagine at WOT with that amount of jitter, you need to tune for the highest timing number you'll see to keep knock away, yet most of the time you won't be at that high timing number!
 

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Thanks very much for the reply Richy, I'll check the belt then in case its that causing it.


Imagine at WOT with that amount of jitter, you need to tune for the highest timing number you'll see to keep knock away, yet most of the time you won't be at that high timing number!
Yeah I see what you mean, that would be highly annoying, what does seem strange though is as RPM's rise its more stable and by 4000rpm with fixed advance its pretty much spot on 10*. So much so that I could then see the spark latency and correct for loss of timing through that. But at idle it was really bad and just all over the place!!
 
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