I have a 2001 road * 1600. It has Cobra Slash cut slip on mufflers on it, stock air cleaner. On one of the mufflers, right behind where it slips on the front pipe, it is turning gold (aft cylinder). what causes this? and also, i do not know if the bike has been jetted when these were put on. it pops and spits when you let off the gas, so i assume it is running lean. any tips on this matter would help out. thanks in advance.