A quick look round the net suggests it would be handy to know which flux was used. It seems high temp stuff, which I imagine is the one used for bikes, needs hydrochloric acid diluted in water
"The only time you run into trouble removing flux is when you haven't used enough of it to begin with, or you've overheated the parts during the brazing process. Then the flux becomes totally saturated with oxides, usually turning green or black. In this case, the flux has to be removed by a mild acid solution. A 25% hydrochloric acid bath (heated to 140- 160°F/60-70°C) will usually dissolve the most stubborn flux residues. Simply agitate the brazed assembly in this solution for 30 seconds to 2 minutes. No need to brush. A word of caution, however - acid solutions are potent, so when quenching hot brazed assemblies in an acid bath, be sure to wear a face shield and gloves."
Not found any articles that say if it will affect the heat markings though.
I imagine Mr Curtis mayknow
