Its on a 1986 ranger 320v. 115 evinrude. it is built in from the factory. Its just below the shifter where the key goes in at. all the old timers i have talked to said it was for fuel mixture, I was just wanting to know. my boat does run a little rough after it warms up. and they ask if i adjust that knob. i was wanted to make sure what it was before I fooled with it. thanks