Question reagarding Spark Plug color.

It's easy enough to take the float bowls off, mount them to a flat plate, run a fuel line to them and check out the float height and if the needles are leaking. Some even just make a sight tube off the bottom drain. That would do the same thing. May want to check that your petcocks are not leaky at the same time. It's good to check all these things on an an old bike.

Hi there I had the same issue; fouling on one plug same as yours. Original Amals with 48K miles on em. Tried to lean it out on clip position but no go. The way I fixed it was to rebuild the carbs with new floats/needles/jets and slides. I reset to middle clip and took a lot of time to make sure that the float seats were adjusted correctly (cant stress this enough as Im sure this was what made the difference). Since then not a single issue.

Hope that helps.

cheers