Take each item from the gear chain in turn, soak it for a little while and then clean the pivots with a nylon brush. Use a magnifying glass to check that they are completely clean with no residual black deposits on the pivots.

Rinse with clean water (preferably deionised water if you live in a hard water area). Dry carefully with a paper towel, finishing with a hair dryer or hot air gun on a low setting.