Sorry to revive a slightly old thread, but I have run into this same "worn out" carb barrel screw problem as well. I check them on every Saito I get, new and used. My most recent 1.25 (used with "only 1 gal" through it) had big flats on this little screw tip and the carb barrel had at least 1mm play easy. This is NOT correct. The barrel should have the least amount of play as possible, otherwise the engine will be able to change it's tune at any time in mid-flight.

If 1/2 turn on the LSN makes a big difference, imagine the needle moving 1mm _directly_ in and out with the barrel slop. Yes the barrel is also opening up a little more air at the same time, but not anywhere close to the mixture ramp that the LSN taper is designed for (thus the spiraled "ramp" on the barrel to start with).

A quick fix is to shim the bolt head so that it stops at a different angel, and play with torquing it down just right to remove as much slop as possible.

-Colin