Ok lets start with the basics.

What a petrol engine needs is fuel, at the right mix with air so that it burns, and a spark to ignite it.
This is about the Bosch K-jet or CIS (continuous injection system), so we will ignore the sparks.

Whatever way you put fuel into your engine (carb, injection) it has to be mixed well with air, and at about 15:1 for standard gas (15 air to 1 fuel).
So the amount of fuel is directly related to how much air is going in, and through the engine, which we will call AIRFLOW.
Airflow isn't just down to how far your foot is on the pedal, but also the RPM, and how efficient the engine design is, and a few other
minor things.

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The injection system, how it works, in as simple a way as possible.

The CIS injects ALL THE TIME. No don't look at me like that, thinking it's going to have all the valves wet, 'cos it ain't true.
Even at 600rpm, each cylinder sucks 5 times a second, so as long as fuel atomised fairly well, it's just fine.....

Early EFi systems were also BATCH or BANK fired, which injects several cylinders at once, until cheaper drive transistors and stiffer laws
made it worthwhile to fire each injector separately. Of course this means you have to inject each cylinder just at the right time, which makes it
quite a bit more complex. So CIS isn't at all that weird.

Now that you don't have to care about cylinder timing and when to pop each injector 'just in time' like EFi systems have to,
the only thing you have to worry about is how MUCH to inject...

The CIS setup simply uses a big flap on a pivot. This flap measures the airflow through the engine. The more air goes through, the more it lifts.
That's somebody's posh law in physics, but it's common sense really. The flap 'floats' in the air flow, and you can change its height (lift) by changing
the shape of the 'tube' the flap sits in. On CIS systems this 'tube' is a cone shape, because the relationship between 'lift' and 'airflow' is not linear
(back to that fancy law again) but a cone will make the flap move in a linear response to airflow (more or less).

OK - if that doesn't make sense, please read it again as it's the FIRST base principle of how CIS works.

Now, if your flap moves in a nice straight way to airflow, it just needs to push on a valve to control how much fuel (petrol) gets injected into the engine,
and it will always be at the right ratio (15:1). EASY !! DONE !!

Yes, nearly, but in the real world it's not quite that easy. Although the job is now mostly done, petrol engines need a bit more than just 15:1 all the time.

Now back to that flap for a minute ....

It's also common sense to see that if the flap is heavy, it's not going to lift as much for the same airflow. That's why big aeroplanes need big wings - right ?
So if you 'push' on the flap a little, it's going to float lower down for the same airflow. So, you can 'balance' where the flap floats, by applying a 'push' (pressure)
to the top of the valve that controls fuel flow, which is exactly the same as pushing on the flap.

This is the SECOND important principle.

So - the flap floats in the airflow, height of flap can be adjusted by applying a bit of pressure to the valve, which allows you to change the fuel mixture.
Therefore if you design a system which provides variable pressure to the control valve, you can change the fuel mixture.

And THAT, ladies and gentlemen is it.....how the CIS works. DAH DAH !! ...

What ??? Oh right... you need it linked to a real actual engine and system..... OK.

[ Please again, stop and re-read if this is not clear. Once you get this principle the CIS system will no longer be a mystery. It's all around how that flap moves]

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The real actual CIS system, as in the wedge.

Right, so, you are stood over the wedge engine bay, and there's wires and fuel pipes EVERYWHERE !!
As the famous book says "Don't Panic !!" We will go through it.

The main batch of pipes (there's er.... 9 ??) are one for each cylinder, 2 that go to a unit at the front,
and one that goes to an injector on the side of the plenum. (did I miss one ?). 6 pipes go to 6 outlets
on the 'banjo', one goes to the middle, and the others go off to the sides.

So - the MAIN unit, with the banjo, holds the big flap (under the rubber cover) and is the 'distribution centre'
for the injection gear. On the side (hard to see of the wedge) is where the fuel comes in (from the pump at the back of car),
and an 'overflow' return back to the tank. On most wedges, the fuel filter is next to the main unit.
(I think early wedges had filter under the floor by the pump ?)

The 6 outlets go to the 6 injectors via the pipe bundle to the engine. The lump on the side of the unit is a pressure control
valve to keep the fuel pressure constant. There are pipes from here which go to - 1) Warm up regulator 2) cold start injector (on side of manifold plenum/air intake).

The main unit is really the flap, the feed pressure valve, and distribution for injectors. That pipe going in the centre top however plays a big role... which leads us to ...

The Warm Up Regulator (WUR - and it's a bit more on many vehicles)

This unit controls the pressure applied to the flap, as I explained further up. So it therefore controls (or 'trims') the fuel mixture.

WUR purpose is to provide the 'choke' function whilst the engine is cold. It lowers pressure on the flap (= richens the mixture) when engine is cold.
On most engines, it's also electrically heated, as you can back off the choke faster than the engine temperature rises. It's normally mounted so it gets
all or some of the block heat, and it's heated from the Ignition On 12v supply.

BUT - as it controls fuel mixture, the WUR units can do more than just warm up adjustment .... some don't, but some do, as in the wedge.

On the wedge, WUR has a connection to the manifold plenum to get inlet vacuum, to do a full throttle enrichment, which 2.8 engine needs.
On some turbo models it has an extra pipe to enrich according to turbo pressure as well (but not all CIS do it this way - see later).

So, back to main 'banjo' unit, it has one pipe OUT from the side somewhere, going to the WUR, and the WUR pipe going back goes to top of the 'banjo' where it
applies pressure to the main fuel valve (and then to the flap), and so controls the fuel mixture.

The other unit you see on the front actually has NOTHING to do with the injection system, it's simply an air valve, to speed up the engine a bit when it's cold,
also part of the choke setup (carbs used to have mechanical connection from choke to throttle to do this). It's mostly electrically heated, but some are bolted to block
(Wedge one isn't really warmed by engine much).

The last bit is the cold start injector, which ONLY sprays when you are cranking the engine. It has a control on the front of engine, which simply switches the injector off when either a) engine is hot, or b) after a fixed time to stop flooding. This is why this unit is also electrically heated as well as heated by coolant, and it looks like
a temp sender but a bit bigger.

And that's it really. The heart of the system is the flap and the (misnamed) WUR.

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Some techie extras , if you're the type that asks awkward questions.......

For the geeks (like me), Yes, some of the things I said here aren't quite scientifically correct, but it's to make description simpler.....OK ??

You will see that the cone in the wedge's main unit isn't actually a true cone... the changes in shape are used to change the mixture across the airflow
range, as height of flap depends on shape of the cone.... so different engines (i.e. by make) have different shape cones. Later CIS with electronic control always DID
have a true cone shape, as the mixture was trimmed in a different way via an EGO, with a different pressure valve on the side of the main unit to do this.

Because the flow through each injector varies a LOT (unlike Efi systems, which turn the injector fully on only for a short time) the injectors are made to provide good
spray (atomisation) over a wide range of flows. It's actually hardest to do this at LOW flow (= idle) so when they wear, the spray typically fails at the idle end first.
This is VERY different from EFi systems. So dodgy idle can be caused by worn injectors.

Little engine, big engine ?? There are different flap sizes in the units. VW golf (Rabbit in USA) has a 65mm flap, Wedge (Ford 2.8), Audis had an 80mm,
and Ferrari and V8 Mercs had 100mm.

Upwards and Downwards ------

Ford, VW, Audi typically had a unit where the flap goes UP, with a connector pipe to the engine. This is what the wedge has.
Some of the Volvos and Mercs had a flap going DOWN, with 'banjo' unit bolted straight to the manifold/plenum. It works exactly the same, imagine the unit
upside down.... yeah OK, not quite that easy, it's built a little differently.

This is why some engines with CIS system look just like a carb, with an air filter in the 'classic' position on the top of the engine..... the main unit is where the carb would be.

Some turbo tricks...

DIY turbo nutters tried turning on the cold start injector for more fuel, but it doesn't work very well, as it's a long way from inlet ports,
and it ain't designed to do that, but you still see that stated around the place. *DON'T* do this !!

When Turbo Technics made turbo kits for the 2.8 engine, they used the original main injection unit, which was 'just' large enough (their words), and they provided a
plastic 'clip on' moulding to fit in the cone to change the midrange mixture, which I think was with original WUR, or you could get a different WUR unit.
I think that's right, but may be wrong there... there was more than one kit type (210 and 240 bhp options ? er...)

[ anyone spot mistakes, please fix, but please keep description as simple as possible so everyone can understand. - thanks ]