Bob, J.Wilson,


I looked through the current ST manual and could not find it, but the carburettor is still of the same design.

There are two small bolts that hold the main needle housing + fuel nipple to the carburettor's body.
Mark the relative position of the needle housing and the body.

This adjustment must be made with the engine stopped.

If the mid-range is, say, too rich, loosen these bolts, so the needle housing can be turned in relationship to the body, make a minute adjustment (turn the housing very slightly) and re-tighten the bolts.
Restart the engine, clear it out by running full throttle for a couple of seconds, reduce to 1/4 throttle and check if it is still too rich, or already too lean.

Repeat, until you get the mid-range set perfectly.