Are you allowing the overspill to exit the muffler. If so, what don't you take the pressure line off when you are filling and that way any fuel that is overflowed doesn't go through the muffler to get contaminated with the exhause residue. That is what I do. I have a pint jar that I soldered a brass fuel line on the lid and use that to catch the overflow. I then dump that back in the jug.