smog test from hell

["William P. Magliocco" <wmagliocco@sprintmail.com>]

Last week I took my '86 5kT (MC engine) to have a smog test done at the
local garage.  

The first time on the dyno it failed.  The mechanic attempted to get the
car to pass by tweaking the timing, then playing with the CO mixture
adjustment on the air flow plate in the CIS box.  Neither allowed the car
to pass.

While it was failing, the car passed the CO and HC portions of the test
with room to spare, only failing the NOx part of the test by about 25%.

The next day, a cat was installed, and the car passed.

All well and good, but now the car has become a dog to drive.  At idle and
to about 1500-2000 RPM one can feel the car missing on a cylinder and also
some shudder due to the miss.  In the 2000-3500 RPM range it seems ok.  I
just drove the car from Phila. to DC over the weekend and it wasn't too bad
on the interstate.

In addition, while stuck in traffic I feel a great amount of heat radiating
from the cat when I drive "cabbie style".  Engine temp seems ok at this point.

The best clue that I have is that the #2 plug gets fouled with carbon.
I've tried cleaning the plugs, swapping the plugs around and even the wires
(keeping the timing sequence proper)...no help.

A few notes:

1-the week before the smog test I ran a emissions system cleaner
("Guaranteed to Pass" from CRC) mixed in with the gas.

2-Installed NEW wires (Beru connectors), Bosch cap, Bosch rotor and Bosch
W7TDC plugs.  Before, I had the "chain store" Bosch WR7DP platinum plugs in
the car, for over a year, with no ill effects.  I know many out there don't
think well of those "chain store" platinums.

3-after the car started stumbling, my mechanic gave me an engine cleaner
made for GM...I allowed the engine vacuum to suck the contents of the can
into the intake and it was quite cloudy upon starting the car up.

4-The dirty #2 plug can be swapped with another plug...the new plug will
then clean itself up in another cylinder and the replacement plug in the #2
cylinder will then get the carbon crust.

5-the tech kept the car on the smog rack for about an hour when trying to
make it pass...would the combination of extended low speed driving (15 mph)
and/or racing the motor to "blow the junk out" cause a problem with the
valves or rings?

Any help is welcome.