On the basis that a picture is worth a thousand words.....
slacken off the handlebar lever;
remove plug from primary drive case;
I have made a small tool from a bit of mild steel
slacken off lock nut, adjust screw, tighten locknut;
re-adjust handlebar lever, a small amount of slack is required;
Apologies for the mediocre pictures.
p.s. I made one error in my previous posting, you will need a 9/16 ring spanner for the locknut.