I assume that you want to know how to remove the air box in order to more easily remove the carbs. Some olders bikes (IE CB 750, GS850 etc,) it is easily done because the designers left enough room to do so. However, on many many other bikes you actually have to sort of wrestle the carbs from between the air box and the engine, I have no experience with the 77 XS750, but, I'd wager that is what you are up against. If so, loosen the carb clamps (both to engine and airbox) then unbolt/unscrew the airbox mounts. At this point it may help to have someone else hold or pry the airbox as rearward as it will go as you yank the carbs out to one side, silicone spray lube can help, good luck.