Being a diesel,someone may have tried using veggie oil or some other "alternative" fuel in it,like used engine oil,ATF,etc.. --or it might just be old grungy diesel fuel..usually diesel tends to have a long life even when its left to sit a long time,but algae and condensation can build up in it..
It is pretty amazing what a diesel will run on for "fuel"..
We had a loader with a 4 cylinder Detroit diesel that started blowing white smoke one day,thought it blew a head gasket--it had sat a good 3 years buried in a old garage ,it ran great after we first got it to run,but a half hour later we thought sure it was burning coolant..
Turned out the fuel tank had a good 4 gallons of water in it,and about the same amount of diesel we put in it...we were amazed it even ran,period!...
We took the fuel lines off and blew them out,and used a gas can as a temporary fuel tank...once it started and ran about 5 minutes the white smoke went away,and you could hear the improvement in how it ran..