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CIS = continuous injection system. This is also known as K-Jet (K = Kontinuous). It's an electro-mechanical system, and was used on a wide variety of cars in the 70s and 80s, from Mercedes to VW, including all "non-special" 911s from '74-'83. The full name was K-Jetronic, even though there were no electronics involved (in most versions).

It uses a mechanical airflow sensor that has a balanced arm holding a plate at one end. *ALL* of the air coming into the engine has to pass through a venturi the plate blocks, and this airflow moves the plate. The arm is also connected to a fuel distributor that has fuel coming in under high pressure (100psi), and is regulated out to the injectors based on a plunger that the airflow arm is attached to. The more the plate moves, the more fuel is allowed to flow to the injectors, which are nothing more than sprung pop-off valves designed to open at 50-60psi.

There are some compensation devices to help cold-starts (very like L-Jet and something like D-Jet), and depending on the exact version of K-Jet used, there are some other devices to enrich the mixture based on engine temp, manifold pressure, and throttle position. These control devices all work by controlling fuel pressure resisting the movement of the plunger in the fuel distributor, thus leaning out the fuel for a given airflow if control pressure is high, and enriching it if control pressure is low. Some very late K-Jet systems used a servo valve to directly manipulate the control pressure under computer control, based on the input from an O2 sensor, so the computer could now control mixture strength.

Acceleration enrichment was controlled by airflow arm balance. When the airflow suddenly increased (like to stomp on the gas), the plate will swing too far from inertia, thus giving an extra bit of fuel, before it swings back. Some very early K-Jet systems had a throttle sensor to reduce control pressure when this happened, but later systems just used a fine balance factor, and just had a throttle closed switch to cut fuel on decel (by raising fuel pressure).

In pure engineering terms, it's a beautifully simple, elegant system.

It's major flaws were that it didn't work well with wild cams, as the intake pulses set up by such cams tend to make the airflow plate bounce. Really violent movement can also make the plate flap around, so fuel delivery over jumps and the like can cause problems, making the system generally unsuitable for off-road use. It's completely incapable of any kind of "sequential" injection, so fuel control at very small throttle openings isn't quite as good as it could be, so eventually it became impossible to meet emissions standards with the system, which is one of many reasons why it was abandoned.