Since a week or two I experienced some trouble with my 1960 dexta. Sometimes all of a sudden it started to run very rough like one cylinder was misfiring. This happened mostly after a few hours of working (mowing with finishing mower). Playing a bit with the throttle lever usually made it go away, but after a few days it became worse.
I own the tractor for about half a year and it has proved to be a very good and happy machine so far, and so I decided to have a look at the injectors. I replaced the nozzles and cleaned everything very well, but too bad it still ran rough as soon as a little bit of power was demanded, even to the point where it became impossible to drive it. I became worried that the injection pump was dying but then I started to read a bit more about engine trouble of other dexta owners and I got the idea to take the lift pump off for a closer look ( allthough it had been replaced shortly after I bought the tractor). It appeared that one of the two one-way valves inside had fallen out of their seat (those damn cheap china parts
) I secured it back into position and.... it purrs like a cat again! Full power no problem 
I hope this little story may help others and avoid complete engine disassemblies just because of a defective (new) lift pump
Cheers!