...Your's is a fascinating question. You need to tell us if your working on an R51(1938-41), or on a R51/2-early R51/3, using early 22mm Bings. The first pattern 22mm and 26mm (R68) Bings use the brass vent you seem to be describing, (and also incidentially the 25mm Zundapp Bings!). The R51 initially used the Amal-Fischer carb, and later the Bing-separate float bowl replacement carb. I don't know if the R51 Bing replacement carb used the brass vent thingie. Black gas sounds like very dirty gas, as Peter implies. Drain the tank and start afresh? You might consider removing/cleaning/rebuilding whatever carbs your using. As an interesting historical aside, the Bings used on the later R51 were designed as substitute for the Amal-Fischers, which were British in design, and which required a licensing fee. The contemporaneous government in power in Germany discouraged payment of those fees. If your still using Amal-Fischers, you might consider removing them for safe storage in the safety deposit box at your bank, and replacing with post war R51/2-early R51/3 22 mm Bings, much easier to rebuild and tune....