engine only idling

[tomainsley]

Here's the story
my pal's engine (an old 6v53 detroit deisel) worked well until a man looked at it, replaced the fan belts and it suddenly stopped working.
I found part of the problem to be that the wires for the start solenoid had come away. These were replaced and it now starts fine and catches first turn.

However, the engine revs will not go above idle at all. If I put the throttle forward then brown/black smoke comes out the exhaust, and there's an occasional knock from the engine. The throttle linkage is fine.
This seems like a fuel problem and the engineer said that there's a blockage in the fuel line. how do i find out where the blockage might be?
I should replace the filters as well while i'm at it. I was surprised that this should happen 'all of a sudden'.
Do any of you have any advice on the matter?
cheers