[Spits] Weird engine idle

[mark.jones at exxonmobil.com]

This started a couple weekends ago when I was out of a cruise.  My Spitfire
1500, with twin HS4s, was working fine until I stopped for lunch and went
to start it again.  Car started running like it was running on 3 cylinders.
I checked and found the plug wire to #3 not "clicking" onto the spark plug.
I changed to a spare plug wire I carry, but still the car didn't run well,
so I just nursed the car home.  At one stop light, in an effort to keep the
car running, I pulled the choke out and the car started running better,
weird.

Back home, I put a new set of plug wires on, no change, I then replaced the
spark plugs, no change.  I did a compression test: 140, 135, 135, 140.  I
started checking the SUs. I found that the rear jet was not retracting all
the way when the choke was fully pushed in.  Took the carb off and found
the choke's return spring out of place, fixed that and tried starting the
car.  The car is running better, but is idling weird.  When the engine is
up the operating temperature, the car will idle at 800 rpm and then, all of
a sudden jump to say 1500 to 2000 rpm while I am sitting, staring at the
carbs.  After 10 sec the idle drops back down to 800 and then after another
10 sec jumps back up to 1500 - 2000 rpm.  Neither carb is flooding, they
seem to be working well.

Any suggestions as to what to look at next?  Distributor?

Mark
80 Spitfire 1500