you can also try a can of seafoam, put a can in tank and its good for 2 to 5 refills, it will break down any gas dapree and clean any clogg's thats not to bad, better fuel flow better gas milage. what year is the bike, if its not older then 4 yrs. then wait on the seafoam. try the diaphram. but like said before, ask what they did, and see if that may of caused the change.
also check the spark plugs, if the carbs, maybe the jets arent set right, it could soak the plugs, causing a change in gas consumption. maybe, but dont qoute me on that.