How to – Rebuild Kubota D850 injectors

The Kubota powered superdream diesel had been having troubles running properly on my new fuel blend. On 95% veg oil and 5% petrol, it ran like this:

Take note of the looping idle and noted inability to idle in the first place.

I attacked many different aspects that could be causing this, air leaks in the fuel lines, blocked fuel filter, dodgy lift pump etc. One of these was poor injector spray pattern. When I used to run it on diesel, it blew white smoke at idle, as seen here:

Attributing this running problem to poor injector spray pattern, I decided to rebuild the injectors. In this process, I also replaced the nozzles (the bits doing the atomisation) , with Monark nozzles. These are known in the MB world for being better when running on WVO, primarily due to the spray pattern at idle. Perfect! Note, that these injectors are similar to many other IDI injectors, so this procedure may apply across different manufacturers.

The problem is, they don’t make these for Kubota injectors. I cross referenced the Bosch nozzles used (DN12SD12) and came up with Monark part no¬†39305027. Perfect.

You will need:

  • Injector Pop tester
  • New nozzles (no point in doing without) as above.
  • Injector adjustment shims (to alter the pop pressure)
  • A vice
  • A correct sized set of sockets and spanners (24mm and 27mm in this case)
  • Some diesel/Veg to test.
  • An ultrasonic cleaner, or a set of picks and lots of patience.

Firstly, Remove the injectors and then clamp them into the vice upside-down.
Crack them off and unscrew the bodies. You will be left with lots of bits:

Split Kubota injector

Only do this one at at time. The injectors wear in a sympathetic way, and they will leak and cause issues if you mix the bits around.

Next, chuck the whole lot into an ultrasonic cleaner. I used a bit of washing up liquid, and a bit of cillit bang, all together with very hot water.

Ultrasonic cleaner

Ultrasonic cleaner





Keep running the ultrasonic cleaner until you stop seeing crud come off them.
Once done, rinse it all off well.

I decided to use a brass brush on a Dremel to clean up the bodies and the insides of the injectors.



Ensure you get into the injector bodies with a pick and make sure they are SPOTLESS inside.

You will then be left with this lovely pile of bits


Put them back together in the correct order, and with the new nozzles in place:

INjector apart x

The picture above is the order they go, its from a Mercedes injector but its the same.

The next step is to set the pop pressure. This is the pressure that the injectors pop and release their injection of fuel. They must be set to the numbers specified in the manual, For Kubota on this engine, its 136-146 BAR. I have set mine slightly lower to counteract the slow injection timing of the fuel.

Connect the injector up to a pop tester, and insert the injector into a jar.


Pop tester

Pump the pop tester up until fuel injects out of the injector (keep hands clear!) Note at which point the injector “pops”. In all of mine they required adjustment (downwards).

Next, build up the pressure to 100 bar or so, then use the wheel on the right to wind up the pressure to just before the injector pops. You want to make sure that just before the injector pops it does not dribble or stream. You want a nice clear boundary between injection and no injection.

If the injectors dribble, they will smoke at idle, and give you the random cracking diesel knock as the fuel combusts incorrectly.

You are also looking for a nice misty spray pattern. You don’t want streams and are looking for lots of atomisation.

If they require adjustment, split them apart again, and insert or remove some shims at the top of the large spring.

(green arrow in the following picture)

INjector apart green

Get them to pop as close as you can to each other. Replace the copper washers on the end and you are left with some shiny new 100% healthy injectors.


Side note РI had trouble with mine set to 125 bar and dribbling. They would dribble a tiny bit until they popped at this pressure. I can only assume this is because the spring pressure was not enough to fully seal the nozzle. I may remove them and set mine to 128-130 bar to prevent dribbling after I get the bike running again.

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2 thoughts on “How to – Rebuild Kubota D850 injectors

  1. What seals the body when you thread it back together? I don’t see any type of internal seal on mine. The reason I removed it was because it was leaking at the threads of the body.

    • They are sealed on the faces of the injector body. If its leaking, they are either overtightened, or there is debris or dirt in between the half’s.

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