IF the choke linkage is smooth and the choke is opening properly AND the plugs still foul (or you see black smoke) in normal driving conditions, then you probably have a non-choke-related mixture problem.
Some possible causes include:
1) Leaky float valve or improper float level.
2) Loose main or enrichment jet.
3) Clogged air emulsion tube orifices (manual calls this the "main vent tube").
4) Bad gasket between main carb body and throttle body, allowing fuel to leak from idle circuit into intake manifold. Loose bolts can do the same thing.
5) Enrichment jet piston stuck in elevated position, allowing rich mixture under high-manifold-vacuum conditions.
6) Overly high fuel pump pressure forcing float valve open (usually not a problem unless high-pressure aftermarket pump has been installed).
In any case, something in carburetor country is probably to blame.
Have fun,
Alan
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