There is a paper ring-shaped gasket between the choke heater and the
carburetor body. These gaskets often deform and then interfere with the
operation of the choke. You should be able to see the gasket all the way
around the junction with the choke heater. If it looks like part of it has
been squeezed into the choke heater, then it needs to be replaced. If your
choke heater still has the OE rivets attaching it to the carb, then you'll
need to drill these out in order to replace the gasket. You can use small
sheet metal screws in place of the rivets to reattach the choke heater.
This will make adjusting the choke much easier in the future. Note that
you'll also want to check the linkage to make sure that it doesn't bind up
and a little cleaning and lubrication would probably also help.
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