Ferrari 400/412 K Jetronic Metering Head Rebuild (Myths and Advice)

I started rebuilding my K Jetronic metering heads and had a long and detailed conversation with one of the suppliers of the rebuild kits (they have been doing this since 1993 and supply dealers all over the world, including Germany).

Their kits include new springs and the port screws o-rings, which should (must) also be replaced.

What was explained to me under no uncertain terms is that ALL screws must me at the same (or similar) adjustment. Bosch allows up to 10% variances.

And yes, the springs and ceramic plates can all be used indiscriminately. It does not matter from which port they are, they are (or should be) the same.

It is a wives tale and old man's bar room chatter (added for the gender sensitive) that "each cylinder is adjusted individually", the fact that is it impossible (on virtually every car) to know what exact air/fuel mixture of each cylinder is.

In fact, up until recently, with direct injection, no fuel system would adjust for each cylinder individually (o2 sensors measure entire banks, or even in some cases the entire engine).

Note: The individual plugs on the exhaust manifolds of our 400/412 are used for trouble shooting, not for air fuel mixture adjustment.

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https://k-jet.biz/product/0-438-100-055-6cyl-alloy-k-jet-fuel-distributor-repair-kit/

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Hello,

unfortunately this is just a myth that has been repeated many times over. The K-Jetronic and KE-Jetronic systems are used in thousands of applications, and those with any type of air/fuel mixture adjustment are based on the entire engine/bank mixture, not individual cylinders.

Bosch manuals clearly state that the metering head has to delivery a "uniform" amount of fuel to all injectors.

Another very clear indication is the fuel delivery test as prescribed by the Bosch manual; referred to as "comparative measurement of fuel".

This test is done to verify that the amount of fuel across all ports is as uniform as possible; Bosch specially made a tester for this purpose, KDJE 7451.

The entire testing procedure is based on trying to get as "uniform" amounts as possible.

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- if adjustment per "cylinders" were done, it would completely invalidate and render this test useless.

Finally, the "individual bosses" in the exhaust are used for trouble shooting, and not individual air/fuel mixture of each cylinder.

From Ferrari/Bosch handbook:

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It is possible that some "tuning" could be done, but the Porsche manual also repeats the same principle:

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These (adjustment) screws, "Einstellschrauben" in German, press against the steel hats, which hold the spring.

From what I understand, the further "in" the screws are adjusted, the richer the mixture - I assume this is due to greater force being directed onto the spring, and as a consequence onto the diaphragm.

Of course the individual ports/screws are adjustable, this is why they are there, but from what I gather the aim is to try to get a "uniform" amount of fuel to all cylinders.




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Here an excerpt from the Ferrari/Bosch injection manual:

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I just spoke with another K Jet specialist, and again it was confirmed: The port adjustment screws only purpose is to ensure even and uniform delivery at the injectors.

When the screw is turned in (again, here the same principle as the idle adjustment screw on the air-flow sensor: RIGHT = Rich, LEFT = Lean.), the amount of fuel reaching the port is increased.

Below you can see the "differential-pressure valve", which is another name for the diaphragm, and how additional pressure on the spring "opens" up the flow to the port. The first image you will see that the diaphragm is "flatter", and therefore less distance/space between the diaphragm and the fuel injection "line" (#3 in the last figure).

When the "hat" is pressed "down", it acts on the spring/ceramic plate and bends the diaphragm down, creating more space, and thus more fuel flow (see second figure).
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The way to adjust these is to measure the amount of fuel delivered by each injector and then adjust them accordingly, not difficult to do.

This is the my first "incursion" into K Jet systems, so I am learning as I go!

I am happy to hear suggestions and the my purpose is to share my experiences and understanding to help others keep our wonderful cars running.

 

Here an illustration I made, the blue arrow is the adjustment screw.
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Another image where the screw is visible and how it presses onto the hat and spring.

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Complete disassembly.

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I actually cleaned them, but they were quite good already.

Yes, the kit is made from Viton and therefore ethanol resistant.

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Correct, offset orifice (here shown at 3 o'clock) is larger.

As recommended by the supplier, I also went over the mating surfaces with 1500 (mirror finish) grit sandpaper by placing it on a glass table. Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login

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I removed the top banjo bolts and counted the number of turns of each of the hat adjustment screws.

They varied between 2 and 3/4 and 4 turns.

This was more out of curiosity as each one will get adjusted by measuring the flow at each individual injector, it will give me a baseline to start with.

I wonder how these were done originally at the factory...

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I am about to finally receive the rebuild kit for the metering heads.

The adjustment screws have all been remove and the o-rings will be replaced.

Contrary to all the folklore, the adjustment is very straightforward.

Before I removed them, I counted how many turns "in" (clockwise) they were set at.

Another way is to measure the depth, but this can have more variances as it depends from where on the port opening you take the measurement.

In any case, this is only a starting point, baseline adjustment, as the exact adjustment will be done once the metering head is mounted on the flow sensor, injectors and fuel pump connected.

Then the process is the following:

1. With all the injectors connected, place each one in a separate container.

2. Measure fuel pressure (5.2 to 5.8 bar) and quantity (minium 850 cm3 per 30 sec) at the return line.

3. Once you know for sure you have sufficient flow and pressure you are ready to set the adjustment screws for each individual port and injector. Here the same applies as the CO adjustment:

Turn to the right =RICH
Turn to the left = LEAN

TBC ...

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Here are the flow rates for the injectors; for idle, part and "full" load.

Does anyone know why full load is showing 3 separate values?

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Here is an example on how the test measurement is done a a Gold K-Jetronic (same principle):

 

And here how the adjustment is done for each individual port:

 

As per Bosch, up a 10% difference between each injector is "within spec" and permissible.

Also useful to know is that 2 full turns of the adjustment screws would result in approximately a 10% difference, so that the adjustment that can be achieved is very limited.

In my case, the differences between each individual port were from 2.5 and 4 turns (so it would mean less than 2 turns and within the 10% tolerance).

The question remains if each injector was individually measured and set intentionally or just "good enough" when assembled at the factory; my judgement has me thinking the latter.

I don't believe at all that they took the time to measure the CO values at each individual exhaust port - the manuals state very clearly that the inspection plugs on the exhaust manifold were used to trace "faults" (not for air/fuel adjustment).