This may be a stupid question, recently bought a 550Kat, whats the little screw adjuster thingy on the back of the row of carbs, slap bang in the middle. Guy mumbled something about an ‘air screw’ it does seem to alter tickover like a choke. In case you havent guessed I’m a mechanical numpty. Cheers
It’s the idle screw, I believe 
+1