Hello everybody!

My JCB 4C's 6 cylinder FORD engine is not working well. Cylinders number 3 and 6 are not working.
All cylinders get fuel but when I disconnect the fuel line (while the engine runs) to 3 or 6, the diesel is flowing but the rpm is not chaning.
Also, the engine smoke a more than usual, grey-blueish smoke.
The valves seems to work okay. In fact, everything seems to be okay but those two cylinders are just not working.

Any ideas what I should do or check next? Get the diesel nozzles cleaned or checked?

All ideas and hints are welcome. Thank you in advance!