TOPIC:

You definitely need to change out the oil before proceeding, but there's no point going any further until you figure out
1. .if you're over fueling and as I remember at some point you had measured 8 bar of fuel pressure. don't remember if you had resolved that yet. Should be around 33 psi or 2.4 bar.
2. Compression- If you have gas in the oil, you've washed down the cylinders. You should put about 1/2 teaspoon of oil down each spark plug hole and then spin the engine over a few revolutions, then do a compression check. If it's over 100 psi, it should run. More like 140 would be ideal, but at 100 it should run.
'3. Spark- As mentioned above, the bike has a wasted spark system. If 1 plug is oil fouled then the spark on the other plug on that coil will be present, but if there's an open circuit wire on a coil, there will be no spark on either plug. Wet plug can spark but usually down in the bottom of the spark plug and not in the combustion chamber and then a spurious spark happens at the spark plug tip and a cylinder fires. Under the circumstances, I hope you have a brand new set of plugs in the bike to rule out bad spark plugs.
4. Have you guys checked the throttle position sensor, just in case it's telling the DFI module that the throttle is wide open and that's causing over fueling?

Replaced throttle position sensor with new one.
Also replaced fuel return line regulator (black thing that looks like small filter) to new one, measured fuel pressure, is within specs.

We are taking the engine apart again. Cylinder head is off now and all the pistons are really nice and clean! On the other hand there is a lot of black residue in the oil pan, probably flushed from various parts of the engine.
We made some test with fuel and valves seem to leak through pretty bad. We will take pistons out and check the rings.

Pulled the head and pistons. Valves are Ok, pistons have some scratches but they are not worn out. Piston rings are gone and we are looking for ring set.
Bottom end bearings are in good condition. Bought one new DFI-box too.

Tuomas wrote: Pulled the head and pistons. Valves are Ok, pistons have some scratches but they are not worn out. Piston rings are gone and we are looking for ring set.
Bottom end bearings are in good condition. Bought one new DFI-box too.

Well we can't say you're not persevering through this. Kudos for keeping at it. Please keep us informed on your progress. Fingers crossed.
Thanks
KB

We had some dark moments during long winter.

We will try new DFI box because there has been some cases where box has opened all six injectors (fully open, discarding firing order) when key has been turned to ON position.
We tried this once already but wanted to double check it with another box.

So far we have not found any specific reason or answer to main problem. From mechanic point of view there is some signs of wear but nothing dramatic. Primary and secondary drive chains have some slack, not bad but some, and we will probably replace them. We are looking for full gasket set too.