I drilled out the pilot air mixture screw adjustment blockoff and reset to about 2 turns out. I had previously rejet to 42 on the pilot jet, so took others advice and thought 2 turns would be OK.
It was WAY too lean on mine. I closed the air mix to just under 1 turn and it starts/idles great now.
On the Keihin clone Youall smoothbore, this is an airscrew. Opening it adds more air to the mix. So closing it to 1 turn enriched the mixture.
The main clue was popping and backfiring. These are signs of lean mixture. I was thinking CDI or something!
It is HARD to get a screwdriver onto this. On mine it is right behind a frame stay. So I used this 4mm screwdriver bit on a short magnetized extension. This REALLY made adjustment easy:
