The Bosch LE-Jetronic fuel injection system is easiest to understand if each of its sub-systems are considered separately. These can be divided into three: the fuel delivery system, the air metering or induction system and the electrical components. Refer to the accompanying illustration for details.
The fuel system starts with the fuel tank in which is housed the electric roller-cell fuel pump. This is protected by a gauze filter on its pick-up side and a full-flow filter on its delivery side. When the ignition is switched on and the starter button operated, the pump supplies fuel to the fuel rail only while the engine is running. The pressure of fuel is controlled by the vacuum-operated pressure regulator which returns fuel continuously to the tank via a check valve or, on later models, a long stack pipe. Although the regulator relies on induction manifold depression for normal operation, it also incorporates a spring-loaded safety valve. The injectors are solenoid-operated and are opened and closed by electrical impulses from the injection control unit; the correct moment is signaled by the ignition control unit based on the information it is receiving from the trigger assembly. Actuating an injector solenoid pulls back a needle valve against spring pressure and allows a controlled amount of fuel to pass from the fuel rail via a small filter, through each injector, and to spray out into the intake port.
The induction system takes cool air from next to the radiator and passes it upwards through the pleated-paper air filter element and into the airflow meter. This device measures the temperature of the incoming air, via a sensor in its intake, and also its volume. The force of the incoming air is used to deflect a sensor flap which converts this movement into voltage by a potentiometer; the information from these two sensors is fed to the injection control unit. The sensor flap is spring-loaded to provide controlled operation and is L-shaped so that pressure variations can be damped out by the movement of the other flap arm in a secondary damping chamber. To permit adjustment of the air/fuel mixture at idle, a bypass duct is fitted into the airflow meter and controlled by a metering screw. By controlling the amount of un-metered air which is allowed to bypass the sensor flap, the screw effectively provides a form of mixture control. From the airflow meter the air passes via a plenum chamber into the intake tracts, in which are situated the throttle butterfly valves which enable the rider to control engine speed via the twistgrip. To overcome the increased friction of a cold engine, a handlebar-mounted lever is provided which opens the throttle butterflies a small amount to give an increased idle speed when the engine is started from cold. A switch mounted on the end of the throttle shaft provides the injection control unit with information on the throttle position. The throttle bodies incorporate air bypass ducts which are controlled by metering screws to provide a measure of air flow adjustment so that the throttle valves, and therefore their respective cylinders, can be synchronized or balanced.
The heart of the electronic system is the control unit which uses microprocessors to collate information about the volume and temperature of the incoming air (airflow meter), the engine speed (ignition system), the throttle butterfly position (throttle valve switch), and the engine temperature (coolant temperature sensor); it then appropriate control impulses to the injectors each time an ignition pulse is detected. The opening time and duration of the injectors can be altered by the control unit to vary the volume of injected fuel and thus vary the air/fuel mixture according to the engine's needs; the surplus of fuel stored in the fuel rail ensures that an adequate supply is always present. To save fuel, the control unit shuts off the injectors when the engine is on the overrun (ie throttle switch in the closed position but ignition system still indicating a high engine speed) until the engine speed has fallen to 2000 rpm or less, when the injectors are reactivated so that the engine does not stall. The control unit also has two safety functions; the first is to shut off the fuel supply if the ignition is switched on but the engine ceases to run or the ignition system fails, and the second is to shut off the fuel supply if engine speed exceeds a set limit, thus preventing engine damage through excessive speed.
All 1985 on models sold in California and all 1986 on models sold in the other 49 states are fitted with an Evaporative Emission Control System to minimize the escape into the atmosphere of unburned hydrocarbons produced by evaporation. When the machine is at a standstill, heat from the sun or from the engine causes the fuel in the tank to expand, thus increasing the pressure inside the tank. If this increased pressure exceeds 0.1 bar (1.5 psi) a pressure relief valve in the tank vent hose opens and allows the surplus gases into the crankcase and via the crankcase breather into the induction system and the engine itself; the tank cap is sealed and will not allow vapour to vent into the atmosphere unless a pressure of 0.2 bar (2.9 psi) is exceeded, in which case a safety valve fitted in the cap will open to disperse the surplus pressure. When the engine is started again, the reduced pressure in the intake tract draws the fuel vapour into the induction system where it is burnt by the engine in the first few seconds of running. An air bleed valve fitted in the fuel tank cap opens at a vacuum of - 0.1 bar (- 1.5 psi) to admit air and replace the fuel consumed as the engine is running.
The engine lubrication system is a semi dry-sump type in which oil is contained in a reservoir formed by the crankcase lower section, thus minimizing the heat build-up and frictional losses that would result from major engine components running submerged in oil. The gear-type oil pump, mounted in a single assembly with the water pump and driven off the forward end of the engine output shaft; draws oil through a mesh filter set in the pick-up and forces it through a full-flow filter element into a main gallery. From here it is fed to the crankshaft main and big-end bearings and into the camshafts, which are hollow but plugged at their rear ends so that oil is positively supplied to all camshaft bearings and then out on to the lobes and cam followers; a supply is also taken from the camshaft gallery to operate the hydraulic cam chain tensioner. The teeth of the crankshaft/output shaft primary drive gears are lubricated by a supply passed through the output shaft itself. All other components are lubricated by splash, the surplus oil falling down into the crankcase lower section. Oil pressure is controlled by a plunger-type relief valve set in the pump housing and a bypass valve in the filter ensures that the oil supply is maintained even if the filter is clogged through neglect, to the point where it cannot pass sufficient oil for the engine's needs. If pressure difference exceeds the set amount, the bypass valve opens and allows unfiltered oil to circulate around the engine. A pressure switch set in the pump housing causes a warning lamp to light in the instrument panel if the oil pressure is dangerously low.