Possible the hydraulic head in the injection pump is worn. Try adding a pint of automatic tranny fluid to the fuel tank. That will increase the viscosity of the fuel some and may overcome the wear in the pump. A diesel mechanic told me that one, worked wonders on Int. DT466 bus engines untill a rebuild could be installed. If you only put limited hours on the tractor you may be satisfied, also, it will provide some lube for the pump absent in low sulfer fuel. Hope it helps     Murray