I would stick a bit of methyl alcohol/methyl hydrate/gas line de-icer in the tank to see if it removes some water which may be causing the sputter. At least it's easy.
don't confuse the folks.....
just tell the product name IsoHeet, or HEET, I prefer IsoHeet, it works best....
and don't just dump it in, you have to run the stuff thru the tank, go for a reallly long ride.
before you go adding that, remove and drain the tank, and drain the float bowls.....and get a clean new gascan and add fresh fuel, and when you fillup add the full bottle of IsoHeet.
do not mix gas additives (i.e. B12, IsoHeet, and SeaFoam in the same tankfull....it will lead to problems, only do one at a time, and then ride tha fuel out, or ditch the fuel...lawnmower gas...)