The little spring loaded pin on the side of each HS6 is used for checking mixture. It lifts the piston a slight amount. Check each carb in turn when fully warmed up. Lifting the pin should cause a slight momentary bump in the idle speed and then the idle should drop to a litle below normal idle speed but continue smoothly - if the mixture is correct. If too lean the idle will drop a lot--maybe even stall the motor. If too rich the idle will rise and stay up. Correct by adjusting the opposite carb--richer or leaner as needed. If the pin is missing, a screwdriver blade under the piston can serve the same purpose. As suggested--make sure all tune-up items are addressed before attacking the carbs. -- Dave