Are you talking about the spring loaded knob for idle mixture? If so, there is an O-ring on the back side. You have to remove the large nut to access. Basically with the large nut off, push the knob in and remove the O-ring and replace. i would highly recommend using a Viton (FKM) material instead of the usual Buna or Nitrile. Viton will hold up longer and cost all of a $1 more. This will be a metric size but you can use inch sizes that are easier to find. You will want to make sure the cross section of the inch sized O-ring is slightly thicker than what you take out. Thicker is OK, thinner will leak. The O-ring should stretch over and snap snug in the groove.
