Just found this thread the other day while trying to fix my friend's Tohatsu 3.5 2 stroke. What the problem turned out to be was the petcock was blocking the fuel flow to a few drops a minute when full open. Unable to disassemble it but there seems to a rubber washer or check valve that only allows fuel to flow in one direction. It was either gummed up or swollen from the ethanol and would allow just enough fuel to idle but not enough to get to half throttle.
Sprayed carb cleaner let it soak and blew it out with compressed air.
Motor now running better than it has in years