Now turn on the spark and turn over the engine for the start, taking care that the throttle is just a little farther open than its fully closed position. If the engine takes a few explosions and stops, you will find the nozzle, or that some part of the fuel piping is clogged which will stop the engine. If the motor gradually slows down, and stops, with BLACK SMOKE issuing from the end of the exhaust pipe, or MISFIRES badly, the mixture is TOO RICH, and should be reduced by cutting down the gasoline supply by means of the needle valve adjusting screw. If it stops quickly, with a BACKFIRE, or explosion at the supply of gasoline should be INCREASED by adjusting the mouth of the carburetor, the mixture is TOO LEAN, and the needle valve.
In all cases be sure that the auxiliary valves are closed when the engine is running slowly, with the throttle closed, as in the above test. If they are open at low speed, the mixture will be weakened and the test will be of no avail.
After adjusting the needle valve as above until the engine is running (with throttle in the same partially closed position), turn the valve slowly in one direction or the other until the motor seems to be running at its best. During the above tests the spark should be left retarded throughout the adjustment, and the throttle should not be moved.
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.