Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(113) Installing the Carburetor.

The carburetor should be placed as near to the cylinder as possible, the shorter the pipe, the less the amount of vapor condensed in the manifold. With multi-cylinder engines the carburetor should be so situated, that is, an equal distance from each cylinder, so that each cylinder will inhale an equal amount of vapor.

The intake opening of the pipe should be placed near one of the cylinders, or draw warm air off the surface of the exhaust pipe in order that gasoline will evaporate readily in cold weather, and form a uniform mixture at varying temperatures.

Great care should be taken to prevent any air leaks in the carburetor, or intake manifold connections, as a small leak will greatly reduce the strength of the mixture and cause irregular running. Always use a gasket between the valves of a flanged connection and keep the bolts tight. If a brazed sheet brass manifold is used, look out for cracks in the brazing.

Leaks may be detected in the connections by spurting a little water on the joints, and turning the engine over on the suction stroke. If the water is sucked in the leaks should be repaired at once. Make sure when placing gaskets, that the gasket does not obstruct the opening in the pipe, and that it is securely fastened so that it is not drawn in by the suction.

Never allow the carburetor to support any weight, as the shell is easily sprung which will result in leaking needle valves.

CARBURETOR ADJUSTMENT. When adjusting the carburetor of multiple cylinder engine, it is advisable to open the muffler cutout in order that the character of the exhaust may be seen or heard. With the muffler open, the color of the exhaust should be noted. With a PURPLE flame you may be sure that the adjustment is nearly correct for that load and speed; a yellow flame indicates too much air; a thin blue flame too much gasoline, and is not the best for power.

Before starting for the adjustment test, try the compression, and the spark. If the compression is poor, try the effects of a little oil on the piston, which may be introduced into the cylinder through the priming cup. It will be well to dilute the oil to about one-half with kerosene. After all trouble with all the parts are clear, you may start the engine.

Turn on the gasoline at the tank, and after standing a moment see whether there is any dripping at the carburetor, if there is, the trouble will probably be due to a leaky float, dirt in the float valve, or to poor float adjustment. Locate the leak and remedy it before proceeding further. Dirt on the seat of the needle valve may sometimes be removed by “flooding” the carburetor, which is done by holding down the “tickler” lever for a few seconds, causing the gasoline to overflow, and wash out the dirt.

If the motor has been standing for a time it would be well to “prime” the motor by admitting a little gasoline into the cylinder through the priming cup, or by pushing the tickler a couple of times so as to slightly flood the carburetor.

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.

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.

Carburetor flooding is shown by the dripping of gasoline from the carburetor, and which results in too much gasoline in the mixture. Flooding may be caused by dirt accumulating under float valve, by a leaking float (Copper Float), by Water Logged Float (Shellac worn off Cork Float), by float adjustment causing too high a level of gasoline, by leaking float valve, by cutting out ignition when engine is running full speed, by rust or corrosion sticking float valve lever, by float binding in chamber, by float being out of the horizontal, by float valve binding in guide, by excessive pressure on gasoline, or by tickler lever held against float continuously.

Dirt accumulated under float valve may sometimes be flushed out by depressing tickler lever several times; if this does not suffice, the cap over the valve must be removed, and the orifice cleaned by wiping with a cloth.

LEAKING FLOAT VALVES should be reground with ground glass or very fine sand; never use emery as the particles will become imbedded in the metal, which will be the cause of worse leaks.

Should the shellac be worn off of a cork float allowing the gasoline to penetrate the pores of the cork, a new float should be installed, as it is a doubtful policy for owner to give the float an additional coat of shellac.

MISFIRING AT LOW SPEED. If the carburetor cannot be adjusted to run evenly on low speed after making all possible adjustments with the needle valve, the trouble is probably due to air leaks between the carburetor and engine, caused by broken gaskets, cracked brazing in the intake manifold, or by leaks around the valve stem diluting the mixture.

INCORRECT VALVE TIMING will cause missing, especially on multiple cylinder engines, as the carburetor cannot furnish mixture to several cylinders that have different individual timing. Look for air leaks around the spark edge openings, and be sure that all valves seat gas tight. Always be sure that the auxiliary air valve remains closed at low speeds, as a valve that opens at too low a speed will surely cause misfiring as it dilutes the mixture.

MISSING in one cylinder may be caused by an air leak in that cylinder.

WATER in gasoline will cause misfiring, especially in freezing weather, as it obstructs the flow of fuel to the carburetor. The carburetor and tank should be drained at regular intervals, and if possible, a strainer should be introduced in the gasoline line.

CLOGGED NOZZLE. Particles of loose dirt in the nozzle will occasion an intermittent flow of gasoline that will result in misfiring. The nozzle should be cleaned with a small wire run back and forth throughout the opening.

CLOGGED AIR VENT in the float chamber will change the level of the fuel, and will either “starve” the engine, or flood the carburetor. The air in the float chamber is a very small hole, and is likely to clog.

HOT FUEL PIPE. If the fuel pipe that connects the tank with the carburetor, becomes hot, due to its proximity to the exhaust pipe of cylinders, vapor will be formed in the pipe that will interfere with the flow of fuel.

DIRT UNDER AUXILIARY AIR VALVE will prevent the valve from seating properly, causing the engine to misfire at low speed.

CRACKS OR LEAKS in intake pipe or gaskets will cause intermittent leaks of air and spasms of misfiring. Old cracks that have been brazed will sometimes open and close alternately causing baffling cases of spasmodic misfiring.

DIRT IN AIR INTAKE will change the air ratio, and the increased suction will cause a greater flow of gasoline. Do not place the end of the inlet pipe in a dusty place, nor where oil can be splashed into it by the engine. Clean out periodically.

“LOADING UP” of the inlet piping in cold weather on light load is caused by the mixture condensing in the intake pipe. The only remedy is to keep the piping warm, or to heat the inlet air.

CLOGGED OVERFLOW PIPE, with engines equipped with pump supply will cause flooding, as the fuel does not return rapidly enough to the tank.