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.
>