The carburetor should now be tested for high speed adjustment, by opening the throttle wide (spark ¼ advanced), and observing the action of the motor. If the engine back-fires through the carburetor at high speed, it indicates that the mixture is too weak which may be due to the auxiliary air valve spring tension being too weak and allowing an excess of air to be admitted. Increase the tension of the spring, and if this does not remedy matters, admit a little more fuel to strengthen the mixture by means of the needle valve adjustment. Do not touch the needle valve if you can possibly avoid it, or the high-speed adjustment, as the fuel adjustment will be disturbed for low speed. If the engine misfires, with loud reports at the exhaust, does not run smoothly, or emits clouds of black smoke at high speed, the engine is not receiving enough air in the auxiliary air valve, consequently the tension of the spring should be reduced. Back firing through the carburetor denotes a weak mixture. Trouble in cold weather may be caused either by slow evaporation of the gasoline, or by water in the fuel that freezes and obstructs the piping or nozzle. In cold weather a higher gravity of gasoline should be used than in summer, as it evaporates more readily, and therefore forms a combustible gas the rate at lower temperatures. To increase the rate of evaporation of the gasoline, it should be placed in a bottle and held in hot water for a time before pouring it into the carburetor or tank, or the air inlet warmed with a torch. The cylinder water jacket should always be filled with hot water before trying to start the engine, and will prevent the gas from condensing on the cold walls of the cylinder. Often good results may be had by wrapping a cloth or towel around the carburetor, that has been dipped in hot water. The cylinder of an air-cooled engine may be warmed by =gently= applying the heat of a torch to the ribs, or by wrapping hot cloths about it. The tank, piping, and carburetor should be drained more frequently in cold weather than in hot, to prevent any accumulation of water from freezing, and stopping the fuel supply. A gasoline strainer should always be supplied on the fuel line, and should be regularly drained. The motor may often be made to start in cold weather by cutting out the spark, and cranking the engine two or three revolutions with the throttle wide open. The throttle should now be closed within ⅛ of its fully closed position, the ignition current turned on, and the engine cranked for starting. This system will very seldom fail of success at the first attempt.