thinwater
Well-known member
Copper alloys, in the presence of water and a trace of salt or organic acid will turn fuel dark due to polymerization (Cu is a catalyst). Most engine manufacturers, ISO, and ASTM prohibit copper tubing for fuel lines for this reason. If the fuel is moving, no problem, but if it is stagnant, this is what happens.
Blog post with engine manf and ASTM links
Blog post with engine manf and ASTM links