Vespas use a simple carburettor with two adjustment screws to set the idle of the engine and the mixture of the fuel entering the engine. Improperly setting either adjustment screw will cause your Vespa to run poorly, not start or smoke badly while running. Properly adjusting the screws will provide you the most power and best fuel economy while driving your Vespa around town.
Pull the rubber grommet covering the mixture screw. Start the Vespa. Allow it to run for 15 to 20 minutes to warm up the engine.
Quickly turn the throttle of the Vespa halfway and release. Turn the mixture screw 1/8 inch with a flathead screwdriver if the engine hesitates when you hit the throttle. Repeat the procedure until the Vespa engine does not hesitate when you hit the throttle.
Loosen the idle screw to lower the RPM of the idling Vespa engine. Continue loosening the idle screw until the engines idles smoothly without over revving. Push the rubber grommet over the mixture screw.
Drive the adjusted Vespa. Make further adjustments if it does not respond or idle after the ride.
Turn off the Vespa engine. Pull the spark plug wire off the spark plug. Remove the spark plug from the engine with a spark plug wrench. A spark plug with a light brown coating signifies a properly adjusted Vespa. Reinstall the spark plug. Push the spark plug wire onto the end of the spark plug.