If you are using an analog throttle servo and have your throttle linkage hooked to the stubby little arm that is standard on the Walbro carb, you can have a very substantial variance in idle speed due to the poor centering ability of the servo and the short arm on the carb. if you use an accessory throttle arm that is at least as long as the arm on the servo and a decent quality digital servo, idle speed will be as close as possible to exactly the same every time you throttle back.