carb is fed from vaccum fuel tap rathern then a manual switch type. Prob not enough vaccum to let the fuel run through....try taking off both the fuel feed line to the carb and the vac pipe on the manifold and suck on the vac pipe. If the tap is ok fuel should run out. It may be that the tap is faulty or that the bikes bore/reeds/crank seals are deffective.
My brother has the same problem and we think is the oil seals on the crank....big ish job:(
martin
December 2008