I had a 78 GS 750....I would suspect a float that has sunk allowing excess fuel to run into one of the float bowls or it could be a faulty float needle....check for a slight ring around the tip of the needle...this indicates that it has failed.....also check your fuel valve....I cant remember if these had a vacuum valve or manual. If its the latter check it isn't on prime also that it isn't leaking.
You may want to check your oil isn't overfilled with leaking fuel seeping into it. If this is the case do not run the engine and drain the oil. You will have to fix the fuel problem before filling with new oil.