yup the arm that when you open throttle on the twist grip moves and obviously opens the carby is a simple plastic arm that is wider at the end on the 18 hp tohatsu which opens full throttle where the 15 hp super is a straight arm and yes if you open your cowel if you own a 15hp super and push the arm you will notice it is restricted and you can with your fingers open more throttle on the carby then it will go with the arm fully open which obviously gives you more throttle so as said go to your dealer buy the part and pay labour if you cant do it.. and you got an 18hp merc super lol confusing isnt it i only found all this out recently after buying my 18hp tohatsu and thought it might help others as i would have liked to know before buying,, happy trails...