Rough Idle

[PRIMROSE]

I have a 2001 Universal M35B diesel with 135 hrs on it.I have not yet changed any original fuel filters,ie water seperator, spin on, or clean lift pump.

 At the end of 2002 season, the engine would run rough when first started below 1500 rpm and then would smooth out after approx one minute.

 Do I have dirty filters or air in system?