the bike has either water in the fuel tank or blocked carb jets or a dodgy fuel tap (some of which need a vaccum to open so it could be this too)as i say the vaccum type fuel tap relies on a vaccum froma pipe that goes to the inlet manifold or carb if this has a split, bend or is perished or fallen off this will also do it.but most likely to be water in the petrol tanki mate drain it out and put new fuel in ok.
adam bike doctor
July 2006