Could just be some bad fuel. Put a 1 oz. of HEET in with 1/2 gal -3/4 gal of fresh gas and see if that cleans up the line. How old is the fuel that you have in them now? Sometimes, you can just get some bad fuel. Today's fuel, like a whole lot of other things, just isn't what it used to be and somehow they charge more for it?
Cub Cadet 123