A quick way of telling if the idle is too rich is to pinch the fuel line at idle. By reducing the fuel flow to the carb, if the idle is rich, it will speed up a little. If it speeds up a lot before quitting, then it's too rich. If it doesn't speed up before quitting, then it is too lean. Ideally, you want the idle to be slightly rich, so pinching the fuel line should speed idle up by just a little, maybe 200~300 rpm. This technique may be helpful when you need to find out when you need to stop fiddling w/ the idle screw.

OS is friendlier with imperfect adjustment. It will run and not quit even if the idle is like 1/2 turn too rich. Saito, on the other hand, you need to be withing 1/4 of a turn.