I had a problem with my 60hp 4 stroke Mercury Bigfoot. Motor would not idle down and not shift into neutral. Local dealership found a tar like residue on the butterfly's on all 4 carbs. They cleaned and rebuilt all 4 carbs and it ran good for a couple of hours but now the same symtoms are coming back. They could not figure out where the residue came from the first time. Anyone ever have this same problem and give any advice?