I thought air screws affect less than 1/8 throttle only, but nothing above. Am I wrong???
No. Just inaccurate.
The air screws in the pilot circuit operate purely via pressure differentials within that lone and separate circuit from the mains and mid-range fuel metering mechanisms of the carb. Less pressure in the carb throat is what forces fuel and air motion. Air and fuel is supplied under outside atmospheric pressure (same as on your skin). A true air screw regulates flow of the air inlet from the air jet. And Idle mixture screw regulates flow of air and fuel after being mixed in the pilot jet emulsion tube.
As long as negative pressure exists at the pilot circuit exit, there will be a contribution from the pilot circuit.
The pilot circuit is the dominant fuel provider when the throttle are at idle position. This provides enough fuel for the engine to overcome internal friction and kept the engine ticking over. This is very small percentage-wise, for throttle settings beyond 1/8 to 1/4 position.
Air screws have diminishing effect on higher throttle settings.
Another way to think of it, is that at idle the pilot circuit provides 100% of the fuel requirements. The adjust screw may vary that fuel volume by maybe 5%. At 50% throttle the pilot circuit may only provide 5% of the fuel requirement. And an air screw change only effects 5% of that 5% contribution. At WOT the pilot circuit may only provide 1% of the fuel requirement, and the air screw could only effect 5% of that 1% contribution.
Cheers,
