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Black stain from poor attempt to clean antique motorycle carburetors

While trying to clean the carburetors of a vintage motorcycle, I've followed a suggestion on a web site and I've boiled them for three hours in an aluminum pot in a solution of water and vinegar [in bulk on eBay or Amazon].
The carburetors have now a black stain that I can't clean and handling them makes my fingers black. Due to the intricate structure I can't just polish them.

Giovanni Motta
hobbyist - Waltham, Massachusetts
2007

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2007

If it was our site that offered the advice, Giovanni, oops, sorry :-)

But I don't think it was, so please tell us where you found that advice so we can investigate.

I believe that what you have done is dissolved a very thin amount of the zinc and/or aluminum into the vinegar, leaving behind the carbon, copper, silicon, or other alloying material that manifests itself as this black smut. A plating shop would use a "desmut" process to remove this, which might be nitric acid, chromic acid, or nitric-hydrofluoric acid depending on the alloy and what they saw.

These desmutting chemicals are not usually considered safe for hobbyists so you might have trouble getting them. If you can afford it, have a plating shop quickly clean these up for you.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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Thanks Ted,

I forgot to say that before the vinegar, I've boiled the carbs in water and automatic dishwasher detergent, the vinegar was meant as a remedy (and it actually cleaned the aluminum pot pretty well, but it did nothing to the carburetors).

Here is a site that suggests a similar method for airplane model engines:
www.airfieldmodels.com/information_source/model_aircraft_engines/maintenance.htm

Can you suggest any shop in CA? Until Sep. I will be in Southern OC. Otherwise, is there anything that I can do at home?

Thanks again and compliments for the site!

Giovanni Motta
- Waltham, Massachusetts
2007

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Thank you for the kind words, Giovanni.

If you cannot mechanically remove the smut, but must do it chemically, I think that you will need to use nitric, hydrofluoric (or fluoboric), or chromic acids. Whether you can do this at home is more a matter of personal opinion than fact. My opinion is that untrained people should not use any of these terribly powerful and toxic acids. Further, it is hard to know which one or which combination will be most effective on your smut.

If you can find, in your hardware store or truckstop, an "aluminum brightener" that contains hydrofluoric acid or fluoboric acid there is a chance that it will do at least a little good. It is still a very dangerous chemical but is probably in a more dilute form that is at least marginally safer to use. Good luck.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007

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It would be a good idea to first confirm that these parts are in fact made of aluminium.

carburetors are commonly zinc based die castings and your mention of '..intricate structure' suggests that these may be.

I would not dunk them in any more chemicals until you are sure.

Geoff Smith
Hampshire, England
2007