K-Jetronic
By Peugeot Pete

The Bosch K-jet is a mechanically controlled Continuous Injection System (CIS). "EFI" refers to systems, that are controlled electronically. The K-jet was introduced in around 1973 (to Porsche 911) when it, together with the L-jetronic, replaced the notorious D-jetronic, which was the first production "electronic" injection system. 

Individual parts described: 

1.Fuel tank 2.Fuel pump 3.Fuel accumulator 4.Fuel filter 5a.Fuel distributor 5b.Air sensor 5c.Pressure regulator 6.Throttle plate 7.Injector 8.Warm up regulator 9.Auxiliary air device 10.Cold start injector 11.Thermo time switch 

2) The pump. It's an electrical roller cell pump. It has a pressure relief valve in case of restriction and a check valve that prevents fuel flow back to the tank. 

3) The accumulator. It has three functions: To damp the fuel pulsation, to maintain fuel pressure when the engine is switched off preventing vapor locks and to reduce the pump's noise. Note that all systems don't have an accumulator at all. 

4) The filter. A replaceable paper filter unit that takes the smudge out. 

5) The mixture control assembly. The pressure regulator (5c) maintains a steady system pressure (about 4.5-5.5 bar) allowing the fuel that is not consumed return to the fuel tank. The air sensor (5b) is located in engine intake air stream. When the engine draws air the plate is lifted. The plate, in turn, lifts the control plunger in the fuel distributor (5a) allowing fuel flow to the injectors. When engine speed increases, the plate lifts higher thus providing the engine with more fuel. 

6) The throttle flap. The valve is an assembly between the air sensor and the intake manifold. The plate is controlled by the accelerator pedal. 

7) The injectors. They are located in the intake manifold, one for each cylinder. The injector has a spring loaded needle valve that opens at a pressure of about 3 bar. It's designed to atomize the fuel completely making it easier to burn. 

8) The warm up regulator. It consists of a heated bi-metal strip, a vacuum diaphragm and a control valve. It's purpose is to increase supplied fuel volume during cold operation and heavy acceleration. 

9) The auxiliary air device. It consists of a pivoted plate, bi-metal strip and heater coil. The purpose is to supply an increased volume of air during cold idling. 

10) The cold start injector. It's usually located in the intake manifold and it sprays extra fuel into the engine at cold starts. It operates only during cranking. 

11) The thermo time switch. It's connected so that it senses engine temperature. It feeds electricity to the cold start injector if engine temperature is low enough and it also limits the maximum time that additional fuel is sprayed. 

Operation: 

The pump supplies a volume of fuel to the fuel distributor. If the engine is rotating the air sensor plate is lifted by the air stream. Thus the plunger in the distributor is lifted and some fuel is fed to the injectors. A lower fuel pressure controlled by the warm up regulator resides on the top of the plunger restricting its rise. If the engine is cold or intake manifold vacuum is low due to acceleration, however, the pressure drops allowing the plunger to rise higher and leading more fuel to injectors. The bi-metal strip in the regulator starts to bend when a heater coil and warmth from the engine itself heats it. The strip pushes against a diaphragm which acts as a pressure restricting valve. Intake manifold vacuum also tends to keep the diaphragm nearly closed. Once vacuum drops, control pressure drops respectively, eventually causing a richer fuel mixture. 

Fuel that isn't fed to the engine returns to the fuel tank. The pump's capacity exceeds highest engine fuel demand. This means that fuel is circulated and filtered many times before consumption. Fuel also keeps cooler since it doesn't absorb heat in the engine compartment for a long time. 

Mixture adjustment: 

The metering plate/distributor assembly has a 3mm allen screw which adjusts the relationship between the plate and the plunger. The screw is located in a vertical tube between the air hose and the fuel distributor. (you'll need an extra long 3mm allen key wrench to turn it). Turning the screw clockwise results in a richer mixture. This has the greatest effect around idle. Turning the screw 1/4 of an turn results in several % of idle CO. Be careful !
Mixture during higher load and RPM can be adjusted from the bottom of the warm up regulator. Under a tamperproof cap there's a 4mm allen screw. Turning it counter-clockwise lowers control pressure. This isn't as sensitive as the CO-screw.
Air flow sensing plate has a cone around it. The design allows more air to pass as the plate rises. Fabricating a cone that has steeper walls will cause the plate to rise higher at a given air flow. Note that nearly all air sensors use different shaped cones, so you'll probably find a cone that gives, for example, a richer top end mixture just by looking. Modifying the cone is risky business, but the sensor frame can be modified to accept changeable cones if mixture can't be made right by any other means.
The standard 4-6 cyl. engine air flow meter (80 mm sensor plate) is capable for about 210 HP. When going beyond this a 110 mm meter from a Mercedes V8, Porsche or a Ferrari is required. I have also heard of installations of dual parallel K-jets. 

Lambda sensor: 

Especially when you have K-jet installed on an engine, which doesn't conform to factory specs anymore, it's essential to keep track of fuel mixture in all driving conditions. Of course, the system could be tuned way on the safe side, but it's not very wise unless gas mileage is not of any concern.The easiest way to monitor fuel mixture is to install a lambda sensor in the exhaust pipe somewhere within 50cm from the engine.
There are three main types of lambdas, some have only one wire, from where the voltage can be measured. Other types include 3 or 4 wires. Two of them are for heating the sensor for quicker operation after cold start (normally you'd have to drive 1 km or so to have accurate readings). On the 4 wire model the voltage is measured between two wires. 

What the voltage tells: 

The range is typically from 0mV to 900mV. The voltage rises as mixture gets richer. The "funny" thing is the voltage leaps from, say, 250mV to 650mV very rapidly at air/fuel ratio 1. It's not the best for fuel economy nor performance, but it's the range where cat-equipped cars try to keep the mixture.
So, you'll most likely to operate just "rich" or "lean". Of course 900mV means loads of black smoke from the exhaust and 750mV is better reading at WOT for an every-day car. On the other hand, the system can be tuned to indicate 0-200mV at moderate cruise and coast.
Monitoring the voltage is most easily done with a common electronics voltmeter, but it's hard to look at during driving especially in the night time and refresh rates might be inadequate. Everybody I know use a led bar, whose instructions can be found here.
BTW: Please don't take the voltage readings as absolute truths. Lambdas are slightly different from each other but the scale they use is the same. 

PP