I am currently restoring a Kubota Z402 engine (used in an Aixam 500SL) that has been treated quite badly. While I have been making some progress I am struggling with the plumbing of the engine. One of the previous owners managed to break off the inlet of the fuel pump. I have drilled it out, added a new thread and created a makeshift fuel inlet until I get a replacement fuel pump. This can be seen on the image below:

High level overview of the fuel pumps

The transparent hose on the right goes to the fuel tank, behind the U shaped fuel line is the fuel pump. The fuel pump feeds into the fuel/water separator (with the orange/red paper filter). Out of the separator comes the U shaped fuel line going into the injector pump. From there it goes to the injectors (currently unscrewed).

I have so far bled the system to the point where I had fuel squirting out of the two fittings above the injector pump (currently screwed on, but previously unscrewed).

As you can see on the left there is a screw screwed into the T-fitting next to the air intake. From what I can tell the previous (?) owner added a T-fitting into the return line of the injector on cylinder 2 to inject fuel - I guess gravity fed - instead of using the broken fuel pump. The return lines are a bit suspect to me as well. Cylinder one (on the right) goes into the injector pump, cylinder two (on the left) goes into the fuel/water separator. Is this normal?

Here is the return line from the left cylinder going into the fuel water separator:

enter image description here

Now another thing confusing me is that there is a lever on the fuel water separator. I can find this lever in the parts description. It is called "lever" without any information unfortunately. It has markings in Japanese (?) that I think mean "Open" and "Closed" - did I "translate" this correctly?

enter image description here

I am getting rather inconsistent results when switching the lever from C to O. I seem to be able to get fuel squirting out of the injector pump no matter the position. Sometimes the T-fitting also squirts out fuel (when I unscrew the screw). I never manage to get any fuel out of the injector lines though.

So my questions are:

  • Does this plumping look correct?
  • Why is one line from the injectors going to the fuel/water separator and one to the injector pump?
  • What does the lever on the fuel/water separator mean?
  • 1
    I don't know if I can "confirm" it, but I looked up on Google Translate what "open" and "close" are in traditional chinese and can tell you where the "O" is is a match for "open" ( 開 ). The one above "C" could be construed as closed ( 閉 ). It seems your reasoning was spot on for at least that :o) Commented May 8, 2023 at 22:34
  • Ah good point, I forgot that in some languages the symbols are more than just a letter and didn't check assuming they also just mean C and O :) Commented May 11, 2023 at 5:46

1 Answer 1


I got the engine running now and the answer to my questions is:

  • Except for the return line of the left injector yes
  • The T-fitting most likely is between the two return lines of the injectors and should go back into the pump for both injectors. As far as I can tell it was custom made in this case. The OEM fuel/water separator only has 2 connectors, one from the fuel pump and one to the injector pump.
  • It normally turns the fuel inlet on or off. For this non-standard unit C means fuel from the fuel pump and O means fuel from the "return line"

All in all this thing is just kinda wacky but I was able to get it running again as it is supposed to be I think.

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