Complaints are received from the field taht the engine will not pull or develop its maximum horsepower, or that it develops good power, but uses far too much fuel. Too often a service man will at once change the carburetor to correct these complaints, but by doing so, he may not be successful in overcoming the difficulty.
It must be clearly understood by all servicemen that when a new engine is designed and developed, the management first decides what horsepower they want this engine to produce at a definite rated speed. The engineering department develops the new engine to pull the required horsepower. In the design there are certain fixed dimensions that never change. For instance, the bore and stroke, the displacement, compression ratio, diameter of valves, lift of valves, diameter of intake passage. The carburetor engineer works out the diameter of throttle bore, venturi size, and provides for means of adjusting and regulating the power fuel mixture ratio, as well as the idle. Now, in service, consider, that the compression, ignition, and timeing have been checked and found to be 100% in this engine. If the air intake temperature and the water temperature is held constant, then the only variable we have that affects maximum horsepower is the fuel mixture ratio.
If compression, ignition, and timing, which are variable, are first properly checked and set to factory specifications, very little difficulty will be experienced in adjusting the carburetor to give the maximum horsepower and economy.
A great deal has been said regarding the importance of engine tune-up and the reasons for being exact in work on engines. The reasons why a carburetor may not function properly when everything else has been checked and set to factory standards will now be covered.
With the present type carburetor construction used on MS carbs, not very much can go wrong with the possible exception that it may foul with dirt. There are only two plactes that are subject to wear-the throttle shaft and bushings, and the float valve and seat. The wear on the throttle shaft and bushings, and the resultant air leak there, results in a lean idle and to compensate for the air leak, more fuel must be turned on for idle. Wear on the float valve and seat results in high fuel level in the fuel bowl and flooding trouble. Both faults can be easily observed and corrected by replacing worn parts. The proper functioning of the carburetor is obtained by a series of holes drilled to exact size and location, which do not wear or change location in service.
Check that: