Others please check that I have this right
I will explain it as best as I understand it- I have only learned it myself last year.
Get a brand-new sparkplug. Run the engine to get it up to temperature. Swap the brand-new plug into the engine. Run said plug for about ten minutes. Kill the engine without relenting on the throttle. Pull the plug and note the color of the white portion- if it is white, you are too lean... dark brown, you are too rich... tawny blond is the color of choice. NOW- once you have determined what mixture of air/fuel you have, and what changes might be in order, open the main cylinder of the carburator and remove the air piston. You will note a brass rod hanging down from it. Remove the throttle cable from the air piston and note the clip that holds the rod in place. Allowing the rod to extend further down will lean out the mixture, drawing it back in will rich the mixture... I forget the exact nature of the adjustment, ie what holds the clip in place with the piston, but there arecut rings in the rod for the clip. Move the clip up drops the rod and leans out the mix, move the clip down will pull the rod up and rich the mix... zb, if you pull a plug chop and find a dark plug, move the clip up a notch or two and re-assemble the thing. Then do another plug chop and see if you have it right.