Depending on if you have a fuel pump or not, It is possible that when going up hill the carb gets above the level of the gas in the tank and therefore no fuel flow. Also I have seen some fuel filters to some weird things the last few years, mainly tied to some of the new emissions issues. Tanks that operate under vacuum, restrict fuel flow, and then a fuel filter that is too fine doesn't allow fuel flow.
Also make sure there isn't any debris floating around in the tank that moves over the hole when going up hill.