F2000 trouble starting

I cannot start my 1987 F2000. I have had it since new. The engine is a 20 HP diesel model D950-FM. It has always been easy to start and has always run well but this year I noticed that it took a bit more to start. It ran well and I cut grass for just over an hour and put it back in the garage. A week later it took a long time to start. It ran well and after an hour I parked it again and shut it off and tried to start it again it did so immediately, so I figured problem gone, but, 3 days later it would not start at all. It turns over vigorously and puts out some dark smoke out the exhaust. I checked the fuel and air filters and they are fine. I depress the clutch and mower PTO is disengaged – the engine would not turn over anyway, if I failed to do this.
What do you think?

The last time I managed to get it started it ran for over an hour. I shut it off and then tried to start it. It fired up immediately. This is what makes me wonder whether it is a fuel supply issue or if it is, why could it start immediately after I shut it off the last time.

Thanks for your help. It was fuel related. We used some quick start to start it and added fuel conditioner. She has been starting fine ever since. This I had never done from day one - will do now though!
Thanks once again.