[TR] Zenith-Stromberg autochoke problem

[Tim Gaines]

I've been working hard this summer to get my 1980 Spitfire 
back on the road, but there have been a number of 
obstacles: brakes, gas tank, frayed wires, and now the 
carb (I think).
The original carb symptom was an alternation from no 
firing to catching and racing and back again, over and 
over.  I checked the diaphragm and found that it was 
brittle and had torn in two places.  No problem; I had a 
replacement.  I was surprised that that didn't completely 
fix the problem.  Since then I have: rechecked that the 
diaphragm is seated properly (3 times), replaced the air 
filter and fuel filter, cleaned and recalibrated the carb 
anti-backfire valve and replaced gasket, replaced needle 
valve in float chamber and checked float height, opened 
and checked carbon canister and checked anti-run-on valve, 
checked fuel pump, checked vacuum and looked for leaks at 
intake manifold and tightened bolts, cleaned small 
passageways in carb with wire, checked that the metering 
needle on the air piston is secured in the right position, 
checked that the damper works (oil, free movement but with 
resistance).  On the autochoke I recalibrated the bimetal 
spring, made sure that the lever moves easily and allows 
the vacuum piston and needle to operate, replaced the tiny 
o-ring on the needle, checked that the stepped cam moves 
properly with changes in the bimetal spring and that the 
choke adjustment pin rests on the cam.  The only thing I 
haven't been able to fix or replace is the vacuum retard 
unit on the distributor, but most on the list don't seem 
to think this is a problem.  I have plugged the vacuum 
line that used to serve that unit.
I have tuned the thing over and over again, and I am 
pretty sure everything is okay (timing, mixture, fast idle 
screw, and throttle stop screw).  Here is what happens. 
 The cold engine starts easily and idles where it should. 
 I have set that with the lower, spring loaded, fast idle 
screw to anywhere from 1500 to 1800 rpm.  As the 
temperature rises and I pull on the throttle cable 
periodically, the rpms go down fine.  Then, when it 
reaches about 1200 rpm, the problem of stopping and 
catching begins.  The engine never stalls out completely, 
but it sounds as if it will on every cycle.  With further 
temperature rise, the rpms step down again and the problem 
disappears.  By adjusting the upper, throttle stop screw I 
can get a nice steady idle at 800 rpm.  Everything seems 
great at this point, but if I pull the throttle cable just 
a little, to get the rpms up to about 1200, the cycling 
begins again.  If I just give it gas quickly, to get above 
1200 rpms, it runs fine though I haven't taken it out for 
a drive.
So my question is, what do I look at next to figure why it 
doesn't like to run at 1200 rpms?  Is it possible that the 
centrifugal advance mechanism in the distributor could 
cause this kind of problem?  I'm pretty much at a loss 
right now.  Thanks for any advice.

Tim Gaines
Clinton, SC
1980 Spitfire
1976 TR6