clamp arrangement. The clamp sits low on the injector body, which allows clearance for the valve bridge operating mechanism. The injector is also known as an offset body because the fuel inlet and outlet are offset to one another. This arrangement allows sufficient clearance between the valves.

Each injector has a circular disc pressed into a recess at the front side of the injector for identification purposes. The identification tag indicates the nominal output of the injector in cubic millimeters. Both the plunger and bushing are marked with corresponding numbers to identify them as mating parts. Therefore, if either the plunger or bushing requires replacement, both must be replaced as an assembly.

The injector control rack for each injector is actuated by a lever on the injector control tube that, in turn, is connected to the governor by mean of a fuel rod. These levers can be adjusted, thus permitting a uniform setting of all injector racks. Basic operation of the unit injector is as follows:

Fuel, under pressure, enters the injector at the inlet side through a filter cap and filter element. From the filter element, the fuel passes through a drilled passage into the supply chamber - that area between the plunger bushing and the spill deflector and the area underneath the injector plunger within the bushing. The plunger operates up and down in the bushing, the bore of which is open to the fuel supply in the annular chamber by two funnel-shaped ports in the plunger bushing.

The plunger descends, under pressure of the injector rocker arm, first closing of the lower port and then the upper. Before the upper port is shut off, fuel being displaced by the descending plunger flows up through the "T" drilled hole in the plunger and escapes through the upper port and into the supply chamber.

With the upper and lower ports closed off, the remaining fuel is subjected to increased pressure by the continued downward movement of the plunger. When sufficient pressure is built up, it opens the flat, non-return, check valve. The fuel is compressed until the pressure force acting on the needle valve is sufficient to open the valve against the downward force of the valve spring. As soon as the needle valve lifts off its seat, the fuel is forced through the small orifices in the spray tip and atomized into the combustion chamber.

As the plunger continues to descend, it uncovers the lower port, so fuel pressure is relieved, and the valve spring closes the needle valve, ending injection. Then the plunger returns to its original position and waits for the next injection cycle.

Injector Timing

Whenever an injector has been removed and reinstalled or a new injector has been installed in an engine, the injector must be timed and the control rack positioned.

The injector plunger is timed by the fact that it meshes with a flat area on the internal rack gear inside the injector body. It is also timed to the fuel control rack - a dot on the gear that is centered between two dots on the injector control rack. Actual effective length that the plunger moves down in its bushing is controlled by the height of the injector follower above the injector body.

To time an injector properly, adjust the injector follower to a definite height in relation to the injector body (fig. 5-26). This will vary according to the size of the injector being used. This dimension is given in the engine tune-up section of the service manual. Current timing pin dimensions can also be found stamped on the valve rocker cover emissions decal. Be certain that you select the proper timing pin gauge (fig. 5-27); otherwise, the engine will run rough and fail to perform properly under load. In addition, continued operation of the injector set at the wrong timing height can result in engine damage.

All the injectors can be timed in firing-order sequence during one full revolution of the crankshaft on all two-cycle engines. A four-cycle engine requires two revolutions of the crankshaft. The sequence for injector timing is as follows:

Figure 5-26. - Timing fuel injectors.