I don’t know which model you have exactly so I picked a random 25 EFI. They do things a little different than Yamaha so my info is not applicable about a filter inside the VST. It looks like they might have a separate HP filter, number 3 in the diagram. I still think the best way to test is with a fuel pressure gauge. There is usually a Shraeder valve someplace; but not always. Are you sure you don’t have some water in the fuel? The rapid decline in run quality that you describe is not usually consistent with a high pressure filter.
Long live American manufacturing!
