Epilog:

It indeed turned out to be the dreaded crankshaft problem, where the key had worn out its slot in the short-nosed crankshaft and the pulley was wobbling considerably. Since the rest of the car was in very good condition and the driver's seat is beginning to conform comfortably to the shape of my butt, I opted for a remanufactured engine (with a long-nosed crankshaft!) and she's now good for another 150K miles, I hope.

Thanks much to all who wrote and offered advice -- this is what makes for a successful list! -- Robbi.

KWS wrote:
> Probably not the timing belt scenario, if it's a '90. Probably is the
> dreaded crank pulley problem. There is a lot of information regarding this
> problem at http://www.miata.net/garage/crankshaft.html . If you look at how
> the timing belt works with the pulleys, I suspect that it could not "jump"
> if it wanted to. If you were describing the problem in a newer Miata (past
> the period of the crank problem), then there might be some other
> explanation.
>
> Since the pulley itself, which bears the timing mark, and the crank angle
> sensor, which detects the cam position, are synchronized by the timing belt,
> a check with a timing light will show the timing to be spot on when the
> problem occurs. Of course, the actual phase of the piston has shifted to the
> ATDC region, so your corrective action helps move the timing somewhat in the
> direction where it should be.
>
> Had you set the timing correctly and now found that it changed "on it's
> own", then a "jump" scenario might be an explanation. I suspect that this
> did not happen and when you noticed the problem, the timing still indicated
> it was correct. You suggested this when you said you set the timing to 18
> degrees BTDC. Had it "jumped", you probably would not have enough range in
> the adjustment to move it that far.
>
> I have the same affliction in my '90. It's on it's second repair using
> epoxy; the first one lasted two years. At some time, I will have to either
> bite the bullet and really fix it (new crank/engine) or get rid of the car.
>
> Good luck, Robbi!
>
> Ken
>
> "Jon Dough" > wrote in message
> news >
>>On Sat, 16 Jul 2005 23:19:37 +0000, Dale wrote:
>>
>>
>>>might of jumped on the timing belt
>>>
>>>"Robbi" > wrote in message
>>>news:zBeCe.45114$rb6.1004@lakeread07...
>>>
>>>>The ignition timing on my '90 Miata (manual 5-speed) needs to be set
>
> w-a-y
>
>>>>out of specs (about 18 degrees BTDC instead of 10) for the car to have
>
> any
>
>>>>pick-up at all. At that setting, the car idles well and drives well,
>
> but
>
>>>>has a very pronounced hesitation/flat spot when starting from a
>>>>stand-still and I need to pump the accelerator to increase the RPM
>
> before
>
>>>>releasing the clutch.
>>>>
>>>>Any ideas out there what might be wrong with it? The sparkplugs are
>
> good,
>
>>>>and so are the ignition wires.
>>>>
>>>>Thanks! -- Robbi.
>>
>>My guess would be timing belt, too. One tooth on the intake side would be
>>several degrees out due to the cam angle sensor driving from the back of
>>that camshaft, one tooth on the crank sprocket (more likely) would be
>>double that amount.
>>
>
>
>