Sticking chokes on SU carbs are fairly commonplace. The jet slides up and
down in the bottom of the carb and it gets stuck pretty easily. It's held
in place by the choke lever at the bottom otherwise it would just fall
out. The center bearing where it rides up and down in needs to be
centered as well or the needle can stick inside of the jet. But that's
not the problem here. You're just dealing with a jet that goes down and
won't go back up.
I'd first disconnect the fuel pipe from the jet to the float bowl, then
disconenct the choke lever and drop the jet out. Inspect it for any
bends. It doesn't take much of an accidental hit with your arm or tool
while you were working on something else on that side to bend the brass
tube body. Visually inspecting it will help a lot here.
If it's fine, then the next possibility is varnish. When a carb sits for
a long time, the fuel will harden into a coating on the walls of anything
it touches. Generally this isn't a problem, but in the jet's case, fuel
that splashes around the outside of the tube from the center bearing can
will settle into that close clearance between bearing and jet
assembly. Let it sit a few months and it will varnish it up pretty
nicely. When that happens, you'll pull the choke, the jet drops and then
sits there, not able to smoothly slide back up.
Again disassemble the jet from the body of the carb, then using a bottle
brush type of nylon brush and lots of carb cleaner, you can clean the
opening in the carb by spraying down from above (remove the dashpots to
get access). Clean that and the jets and it should slide up and down very
easily.
Lastly, make sure the choke linkage isn't bent. It doesn't take much of a
twist in this light metal link to cause binding.
My home page has some info on working on the SU carbs for this very
problem. A good photo of the area that is affected is at:
http://jarmac.picarefy.com/spitfire/jet.jpg
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