Instantly or after a long run..? Instant, it could be condensation in the exh. system. If after a long run, it could be coolant that has leaked thru. The black is soot that collected along the way.
Does the water/coolant level drop..?
As far as rich....determine that by how it runs & look at the plugs..!
Plugs are tell-all...

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I'm leary of putting "cleaners" in an un-familiar tank/fuel system. Not sure what it might loosen up in the bottom of the tank. I have a problem...whatever it "cleans" has to end up in the carb.

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