After some work and new parts (starter motor, battery, headset and injectors) we'd managed to get our tractor running. Then it started conking out, and now become hard then impossible to start once again. When it first stopped starting, there was lots of fumes from the exhaust but it just couldn't quite click into life, but now there's relatively little white smoke when turning the engine over. We've bled the fuel system, fuel getting to all the injectors though noticeably less flow to the fourth than the other three. Fair to say we've reached the limits of our mechanical ability, and getting a bit frustrated! Any ideas?
I should add that the engine has also developed a more fundamental flaw, in that fuel (diesel and oil) is dripping down the bottom of the exhaust - but I don't think that should be preventing it starting?