'87 8v - Low idle/light backfire - need HELP

[PK Commercial" <pkcommercial@mindspring.com (PK Commercial)]

1987 Scirocco 8v w/ 153k

Symptoms: low idle @ 500 rpm, light backfiring during deceleration in gear


I would describe it as a light backfire as opposed to the heavy 'bang' sound
so often associated with the term 'backfire'.  This is not the sweet
burbling of an aftermarket exhaust here, but a pronounced 'pop, pop-pop'
that indicates an excess of unburnt fuel escaping in the exhaust and hitting
the manifold.

With the low idle, I noticed this about 6 months ago.  It seemed to just
drop down one day.  I have checked the timing and noticed that it is well
advanced to the point that you can't see the mark.  If one were to audibly
set the timing so it read about 800-900 rpm, it would be about 15deg
advanced (with, of course, great knocking).  So to bring it within the
timing range of 4-8 deg BTDC, it will take the engine to 500 rpms.


What I'll rule out, but you can offer a different viewpoint:
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Timing Belt Slipped - the power of the car is unchanged.  Acceleration and
cruising performance have not altered one bit.

Vacuum leak - checked system and none found.  It's tight.

Fuel system - well, the backfire tells me it's getting enough fuel.

02 sensor - disabled and same problem; backfire starts once manifold is warm


What I think it could be:
-------------------------------

Control module - which as we know only controls the fuel delivery.  My
concern is based upon one thing in particular - the fact that after about 3
minutes of driving the rpms jump down from about 750 rpm to the 500 rpm I
noted earlier.  This is in cold or hot weather, so I will disassociate the
cold start system here.

Distributor - could the centrifugal advance be stuck so that it's running in
an advanced position at all times?  I've rebuilt them before, but would like
an opinion as it's never fun.


Thanks for reading and any thoughtful input, folks.  Please respond to my
email address as well since I get the Digest version.

-Phil Kerschner