From Rob : A yellow flame is a product of incomplete burning of the gas. It sounds to me as if the burner heads need to be cleaned. This can be accomplished by rermoving the burner head with a pipe wrench and taking a very small round wire file and reaming out the orfice holes. You aren't trying to enlargr the holes, but clean then of any built up impurities that may be in the gas. It produces a hard crusty sort of thing and can completely block some holes. This should solve the problem.