Dave,
I try to adjust a YS91FZ for a friend and found something that bugs me. I've had a couple of 140 and am 'relatively' familiar with the YS.

The instructions indicates that to lean the low end, the needle should be turned CCW (reverse of the normal way).

Looking into the carb barrel, the screw is closing a hole if turned CW. This hole is only efficient at idle, when the flap is closed, and it controls the 'compressed' air/fuel mix coming from the crankase and going to the chamber.

How can closing this hole make the low end richer? Don't get it!
Thanks

Ben