The Classic Machinery Network

I just noticed this and I had the same symptoms on the D1105 kubota in my kubota FZ2400 mower when I bought it (cheap...) that under heavier loads it seemed to fade away although it was generally underpowered and smoking anyway.
After changing out all the filters, on that with no change it was also one of the pumping elements out in the injection pump. I found the seal kits on ebay and did it myself and now it hums happily away and gives me more time to get the jobs done on stuff I like instead of fixing the lawnmower mid summer.
Because the pump camshaft and rack stays in the crankcase, and you lift out the pumping element cylinders, its more diy'able than a normal injection pump (although I did the CAV dpa on my jcb myself also). You can leave part of the pump upper in situ that controls the injection quantity but still remove the elements themselves.
Think it might be fairly common on these little kubota diesels.

Yeah the injection pump is fairly straight forward on the Kubota v2203 DI and i repaired it myself. I ordered and replaced all 4 plungers when i had it opened. The only thing to watch out for is the alignment of the timing mark on the internal injector chamber gear to the stop lever which slides over and back. Also keep it really clean!!. There are a few videos on youtube of guys dismantling these injection pumps. Engine was like a different beast altogether after the repair. another common issue in the kubota injection pump is the internal chamber springs can crack and then u also get the bogging down under load symptom and blue smoke. The reason for this is that the spring is designed to draw the plunger back down after it is pushed up by the cam shaft in the crank case. if the spring is cracked, that injector plunger only partially operates. A quick test is to remove the injector feed pipes from the top of the injection pump and crank the engine over. each injector chamber should blast diesel up in the air at least 6 inches.

Same. I was sweating the whole time doing the dpa, but the kubota pump seemed really simple, just a matter of cleanly-ness and being sensible.
The kit came with new o-rings & return springs, and Ive seen reports of a broken spring prior also so I changed out mine also while it was apart. I had noted warnings also about breaking the ears on the stop lever arm when putting the pump body back in the crankcase. I didn't put new elements in it but springs/seals and a thorough clean sorted it out.
To test post rebuild I checked pop-off pressures on the injectors & put the ends of them into some graduated tubes and cranked the compression-less motor over for a few minutes with periodic stops to avoid cooking the starter and checked they pumped the same quantity of fuel out over the period, not super accurate but its how a real phasing and calibration rig does it.

Just a note that i forgot to add on the assembly of the injection pump. The plunger piston has to be aligned in a such a way that the helical slot in the plunger is lined up with the inlet port on the the barrel otherwise no diesel will get injected. The internal injector chamber gear will then be able to rotate the plunger piston to either shutoff or enable fuel to reach the injector head nozzles.