[alfa] Marelliplex Timing, possible pre-ignition

[Mark Denovich <denovich@xxxxxxxxx>]

I have an '86 Spider with Marelliplex ignition installed.   What
experience I do have tinkering with distributors and ignition timing
is with Bosch units, so I need to confirm a suspicion or two.

Under very specific circumstances (3000 rpm, and very light throttle)
on top of the regular exhaust note I get a "tat-tat-tat-tat" noise... 
The "tat" is like a marble hitting a pie-pan.   The noise does not
come from the engine bay, but from under the car.    It is not a
rattle.   I have confirmed there are no exhaust leaks, and the
manifold is well sealed to the head.   More throttle, or no throttle
and the noise goes away.   This is on 93 octane gas.

The 'Plex dizzy I have has been modified to employ two of the lighter
weight springs (I think stock is one heavy, one light) which results
in more advance earlier.

So I'm guessing I'm experiencing pre-ignition.   The advance coming on
at 3k and the just cracked open throttle causing a lean condition, and
the resultant ping.

Assuming my diagnosis is correct I need to retard the ignition
slightly.  Correct?
(I wanted to ask before I started fooling around.)

I'm trying to think of other possible culprits...  I suppose anything
that causes a lean condition around 3k could also be suspect.  
Anything else I should look for.

Thanks,

--Mark
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