Not a YS specific reponse I know, but I have had similar problems with a Saito 150. It would run fine on the ground, idle all day nice and slow, bu in the air it would quit as soon as you throttled back in say a loop. It was really frustrating until I one day tried richening the idle slightly. It didn't seem too lean before and you couldn't detect any change on the ground but it stopped it quitting in the air.

I would try that, I can't garuntee anything of course and my experience wasn't on a YS but it might be worth a try. Unfortunatley my set up which sounds identical to yours (L, WC30, hatori pipe etc) works perfectly so there must be something out of place. If you change the fuel lines (always a good idea) don't forget about the one inside the tank...