The complete Garelli touring model motor cycle, which closely resembles the sports model on which the 350 c.c. class in the Grand Prix was won, the chief alterations being in the handle-bars and the special double vaporising chamber carburetter fitted.
Diagrammatic section of the Garelli engine, showing the two pistons mounted on a common gudgeon pin and the single connecting rod. Observe the common combustion chamber.
Side and front views of the Garelli engine, showing the enclosed Mea-Garelli magneto and the change speed gear.
In view of the triumph of the Garelli machine in the 350 c.c. Grand~ Prix some details of this Italian two-stroke cannot fail to interest our readers.
The engine follows similar lines to those of the old Lucas two-stroke, which, as some readers of The Motor Cycle will recall, was fitted to the valveless car. In appearance the power unit is extremely neat and in many respects sets an example which British manufacturers could follow with advantage. We refer chiefly to the manner in which the magneto is housed and to the unit construction of gear box and crank case.
Common Combustion Head.
The two cylinders (50 x 89 mm.), are arranged transversely across the frame, with an air passage between them; they have a common combustion head. The two pistons are connected by one gudgeon pin and transmit the power by a single connecting rod.
Primarily induction is via the crank case in the usual way, and it is controlled by the right-hand piston. The gas transfers at the bottom of the left-hand cylinder through a ring of ports in the wall, and the burnt gases make their exit through ports uncovered by the right-hand piston. Only one sparking plug is used, and the impulse effect is identical with that of a single-cylinder two-stroke engine of the conventional tune.
Technical Details.
A ball bearing of large diameter is employed in the connecting rod, this being somewhat unusual. The crankshaft itself is also mounted unconventionally, for the shafts float in compression-tight glands, the crank discs being carried on ball races mounted on fixed flanges formed on the base chamber walls.
In the case of the racing machines used in the Grand Prix the particular type of Zenith carburetter employed had two vaporising chambers, one for each cylinder and a common float chamber.
Another interesting feature of the machine is the arrangement of the Mea-Garelli magneto, which is cylindrical in shape and is carried in a housing, forming an extension of the crank case. The use of this type of magneto on a two-stroke engine is to be commended, since the ignition range is controllable by rotating the magnets in relation to the armature, and not merely by turning the contact breaker cams ; by this method the full efficiency of the magneto is obtained at any degree of retardation. A similar extension on the other side of the crank case encloses the clutch and gears, giving two speeds (4.8 and 8.5 to 1). The clutch is of the metal disc type, consisting of plate steel and bronze plates. This arrangement allows the use of but a single chain, which runs at low speed.
The Garelli is manufactured by La Societa Anonima Moto Garelli, 287, Casella Postale, Milan, Italy.
The MotorCycle July 20th, 1922.
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