Normally we clip the hook off that spring, and just let it float there to maintain a little sideways pressure on the throttle shaft to prevent wear caused by vibration. That spring is only used as a safety to close the throttle when the carburetor is used on power equipment such as chain saws, etc. We don't use them for R/C airplanes, as they are far too strong for the throttle servo to be battling to overcome.

You might try an APC prop. They are more efficient than wood props I have found, and they weigh a touch more which helps the idle.

AV8TOR