IMHO, if the car will start when cold without the choke being pulled out,
then the base idle mixture is too rich and needs to be readjusted. The
reason why it won't stay running with the choke pushed in is because the
engine still needs a fast idle when it's cold, which you're providing with
the gas pedal.
Re your lack of increased idle speed when the choke is pulled out - that is
a purely mechanical function, with the cam mechanically actuating the
throttle lever to increase the idle speed. So either that linkage needs
adjusting, or you've got some other carb problem.
However, before messing with the carb, I would doublecheck timing and points
gap just to make sure nothing has wandered ...
Then I'd get the engine warm and re-set carb balance, idle speed and mixture
(making sure that the choke linkage is not engaged.) once you're happy with
that, then work on getting the choke fast idle adjustment right.
Re stumbling from a standing start until the revs get going ... One thing to
check for would be a torn carburetor diaphragm.
Finally, I have a Colortune spark plug and like it a lot ... I never could
get the "lift the piston" technique to work well enough for me.
-Graham
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