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
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