A cross-section of a Stromberg carburetor is shown at A in Fig. 42, and it is fitted with many little refinements such as a needle valve adjusting screw, a gasoline strainer, drain cock, hot-air horn, etc. A view of the carburetor complete is shown at B.
The Electric Ignition System.—The only type of ignition system used on motorcycle gasoline engines is the jump-spark.
As in stationary gasoline engines there are two kinds of jump-spark systems used on motorcycle en gines, and these are (1) the jump-sparkeoil system, and (2) the high tension magneto system.
On the cheaper makes of motorcycles the jump spark coil system is used, while the better makes are equipped with the high-tension magneto-system. The jump-spark-coil system and the high-tension system used on motorcycle and other mobile engines work on the same principle as those described in connec tion with gas engines.
The Ignition Timing System of a a jump-spark igniter is used on a single cylinder engine a timer, or interruptor, as it is called, is used to make and break the primary circuit, as previously explained, but where two or more cylinders must be fired successively a revolv ing timer, or distributor, as it has been named, is employed.
A distributor consists of a disk around which are fixed as many metal contact segments as there are cylinders to be fired, and each segment is connected by a wire with a spark-plug, as shown at A in Fig. 43.
A metal contact arm is pivoted to the center of the disk and the former makes contact with some part of the engine, that is, it is grounded, so that it will form a connection with the spark-plugs.
The contact arm is turned by the camshaft and makes one complete revolution to every two revolu tions of the crankshaft. Now every time the arm makes contact with one of the segments of the disk a spark takes place between the electrodes of the spark-plug that is connected with that particular segment all of which is clearly shown at B.
A lever is attached to the disk of the distributor and goes up to a control lever on the handle-bar, so that the disk can be moved ahead or set back, when the spark will be advanced or retarded, as the case may Kinds of Lubricating Systems.—There are two schemes used for oiling motorcycle engines, and these are (1) the splash system and (2) the force feed sys tem.
The Splash, System—This is a simple system for oiling the working parts of a mobile internal com bustion engine but it must be looked after rather closely. In this system the oil is (1) poured into the crank-case through an opening called a breather; (2) it is fed into it from a tank by gravity, or (3) it is forced into it from a tank by a pump.
The oil is then splashed into the cylinder and onto the connecting rod bearings by little scoops, or spoons, fixed to the lower ends of the connecting rods which dip into the oil. It is shown in Fig. 44.
The Force Feed System.—All the finer makes of mobile engines for all purposes use the force feed sys tem of lubrication, or mechanical oilers as they are sometimes called.
In this system the oil is put into a well in the bot tom of the crankcase, or sump as it is called, and from there it is forced by a pump worked by the en gine through an oil tube above and parallel with the crankshaft.
From this tube the oil is forced or flows through ducts, or holes, in the crank-webs to the crankshaft bearings, on through to the crank-pins and through hollow connecting rods to the wrist-pins. The pis tons are kept lubricated by the oil that runs out and which is thrown from the lower ends of the connect ing rods. The whole scheme is shown in Fig. 45.
Cylinder Cooling Schemes.—There are two kinds of cooling systems for keeping down the heat of ma torcycle engines, and these are (1) the air cooling sys tem and (2) the water circulating system; then there is (3) a combination of these two systems.
In air-cooled engines the cylinders are either cast with thin parallel rings around them or else have copper rings fixed to the outside walls as shown in Fig. 46. When the engine is running the rings get hot and the heat is radiated by them into the air.
As the water circulating and combined air and water cooling systems are seldom found on motor cycle engines but are used on every make of motor car engine, with one or two exceptions, these systems will be described under the next section which treats of motor car engines.
How the Muffler is Made and muf fler 1 of a motorcycle engine must be small and light. To provide such a muffler six or more baffle plates with holes drilled through them on opposite sides are bolted together and to a head, as shown in Fig. 47, and the exhaust pipe is screwed into the latter.
Now when the exhaust gases are discharged from the exhaust pipe or manifold, the gases pass through the holes in the first baffle plate, then the next, and so on until they reach the outside air. This arrange ment of plates makes the streams of gases change their direction and breaks them up half-a-dozen times before they reach the outside air, and by this time the force of them is quite effectively dissipated.