Start the engine and run it at idle. Pinch the fuel line between your fingers to stop any fuel from going to the carb. If the engine immediately loses revs then it's too lean on the idle mixture screw. Wind the screw out 1/8 turn and try the pinch test again. What you're aiming for is that when you pinch the line the engine should continue running for a short time while very slightly increasing it's revs before starting to die off. A very slight increase in revs means it's just barely on the rich side at idle.

If the idle mix is too rich then the revs will rise fairly considerably for several seconds when the fuel line is pinched. In that case turn the idle mixture screw in 1/8 turn and repeat the test until you only have a slight rise in revs.