The petcock, or fuel valve, on the CB500/550 is a problem. Uncharacteristically, it one of very few real weak points on the bike. Most of the reason is the fact that it is made of an aluminum alloy that has even more zinc in it than do the carburetors, and that's saying something. Basically, it's a Cracker Jack toy masquerading as a petcock.

Zinc is a very poor metal. It was in common use when casting technology was not as advanced as it is now because the metal is one of the freest flowing, thus not requiring the most sophisticated molds. Some manufacturers, particularly carburetor manufacturers (Keihin made the petcock and the carburetors), continued to use zinc long after better casting techniques were in use, probably from a combination of cost reasons and good old inertia.

The softness of the metal is why the petcock's brass nipples keep coming loose. We mechanics used thread locker to fix them in place, and then instead of coming loose they would just slowly weep fuel. Additionally, the petcock's sediment bowl, due to the flimsy metal, has very sketchy threads that seem ready to let go at any time, with frightening results. Watch the petcock closely.