The Gasoline Motor

by Harold Whiting Slauson

Were the gasoline fed directly from the fuel tank to the needle valve of the carburetor it is evident that the rate of flow of the liquid would depend, to a large extent, upon the amount in the tank and upon the position of the car. This would cause each charge to differ in the proportion of gasoline vapor to air, and it is hardly probable that the motor could be run at all under such conditions. In order that the pistons may suck the gasoline from a level that does not vary with the amount of fuel in the tank or the position of the car, a separate compartment is provided in the carburetor. This is known as the "float chamber," and it is from this compartment that the gasoline passes through the needle valve into the vaporizing or mixing chamber.

A cork or hollow metal float is placed in this float chamber and is mounted on a lever connected with a valve located at the end of the gasoline feed pipe. As the gasoline is admitted to the chamber, the float rises and closes the valve controlling the flow of fuel. As the gasoline is sucked through the needle valve from the float chamber, the float in the latter lowers, and the fuel is again admitted by the opening of the above-described valve. The float and valve are exceedingly delicate in their operation and the gasoline is thus kept at a constant level in the chamber under all conditions of the car and tank.

The stem upon which the float of some carburetors is mounted is sometimes threaded and provided with a nut by means of which the float may be raised or lowered. This furnishes an adjustment for varying the level in the float chamber and determining at what point the flow of gasoline shall be cut off by the automatic valve. The float is supposedly properly regulated when the carburetor leaves the factory, but the stem may become bent or the carburetor may be applied to a motor other than that for which it was originally designed. In either of these events, it may be found necessary to raise or lower the float before the proper level of gasoline can be maintained in the chamber.

If the float is too high on its stem, the gasoline control valve may not be operated until the fuel overflows in its chamber. This is known as a "flooded" carburetor and produces a rich mixture which will ultimately prevent the proper operation of the motor. Turning down the gasoline supply at the needle valve will not remedy this, for the fuel will reach the vaporizing chamber by another route. A flooded carburetor often gives trouble, and while it may be remedied easily, the amateur may experience difficulty in locating its source.

As soon as it is discovered that a carburetor has become flooded, the needle valve should be tightened so that no gasoline can pass through it, and the motor should then be cranked. This will serve to evaporate the excess gasoline in the float chamber and reduce the level to the point at which it will not overflow. The exact number of turns and fractions of turns through which the needle valve nut was moved should have been noted in order that the valve may be reset to its original position after the surplus fuel has been "cranked out."

A float that is set too low on its stem will close the fuel supply valve before a sufficient amount of the fuel has flowed into the chamber, and will form a "lean" mixture at high speeds of the motor—even though the needle valve should be opened wide. The obvious remedy for such a condition is to raise the float until the gasoline will be maintained at the proper level. If there is no nut and screw adjustment by which the float may be raised, the arm to which it is attached, and which is connected with the valve, may be bent slightly.

But the motorist should not "jump at conclusions" and assume that the float is improperly set the moment the carburetor begins to flood or the motor appears to "starve" at high speed. The first condition may be caused by a piece of dirt or other foreign matter that may have become lodged on the valve seat and prevented the valve from closing when the gasoline reached the proper level in the float chamber. This will produce exactly the same results as will a high float and is a trouble that will more often occur in the average carburetor.

The difficulty may generally be remedied easily by draining the gasoline from the float chamber after the valve in the main supply pipe has been turned off. The offending foreign matter will generally be carried with the gasoline as the latter is drained, and the valve in the feed pipe may again be opened as soon as the drain cock is shut off. If this fails to remedy matters, it is probable that the difficulty lies with the float.

A clogged gasoline pipe or dirty strainer will produce the same effect on the operation of the motor as will a float that is set too low on its stem. When the motor seems to starve at high speed, and it is evident that there is sufficient gasoline in the tank, the union should be disconnected at the point where the feed pipe joins the carburetor. If there appears to be an ample flow through this pipe when the main valve is opened, it is probable that the stoppage has occurred in the strainer. If the flow through the main feed pipe is not free, however, it is possible that the vent hole in the filler cap on the tank has become stopped or that the latter has been screwed down too tightly. In the gravity feed systems, some method must be provided to allow the air to flow into the tank to replace the gasoline fed to the carburetor. If there is no hole in the filler cap, the latter should not be screwed down so tightly that an airtight joint will be formed.

Probably the simplest method of determining whether the trouble lies in a low float is to prime the carburetor and to observe the ease with which this can be done and its effect upon the engine. Nearly every carburetor is provided with a "flushing" or "priming" pin by means of which the float can be depressed so that the gasoline chamber will be filled rapidly to a point above its normal level. This is useful in starting, as the desired rich mixture is quickly obtained without an undue amount of cranking. If the carburetor flushes easily, it is evident that there is no serious stoppage in the pipe. If this easy flushing is followed by good running on the part of the motor, and if this, in turn, is succeeded by gradually-diminishing impulses indicating a weakening mixture, it is quite evident that the float is preventing the flow of the gasoline at the proper time.