chewy

I have a farmall 856 that runs good. The last year or so it has started throwing diesel out the exhaust, to the point that it is covering about 1/2 the hood. I was told to run it hard and it would clear up. I did run it hard and it did clear up, but I have been feeding hay with it and it has started throwing diesel again. Can someone tell me why, it did not do this before. Should I take the pump off and have it checked?
Thanks

It's just not burning the fuel that's why it cleans up when you run it hard

Is a functioning coolant thermostat present, and does it "warm up"?

Well when was the last time ya had it tuned up ????? yea diesels need a tune up . The valves need reset and done wright . When was the last time ya had the fuel system done ?? and like was mentioned is the thermostat working ?? the injectors maybe slobbering due to this fine fuel and lack of working the tractor . That is why we run gassers for chore work . They always start they warm up faster and cost less to tune up . So with out looking at the tractor and pulling the injectors and testing them this is the best i can give ya.

The oily spots are a direct result of the engine operating cold. It is called Wet Stacking. As a Hydrocarbon fuel is oxidized, it forms water and carbon dioxide (and other trace chemicals).
The water is burnt hydrogen. H2O. The CO2 is burnt Carbon.
A diesel operates on 100$ air filling the cylinders. The amount of fuel is portioned out to make it do the work asked, and when putzing around at part power, the extra air is just passed on through the exhaust. The extra air reduces the operating temperature such that the water and soot condense in the exhaust system. This combo is what is spitting out the exhaust pipe. The tractor was designed to operate doing work at near capacity for hundreds of hours. So as it is, and as it is being used, it will keep doing it.
Putting a sweeping right angle pipe on the exhaust will divert some of it to the side.
Insulating the exhaust so it gets hot enough to reduce condensation would work when it was not worked hard, but would overheat the pipes if used for hard work. What to do?
New diesels have pollution control catalytic converters, liquid injector, and computer controlled injection and almost eliminate the issue.
Old gasoline and LPG tractors don"t do it because the exhaust gets hot and no extra air.
Work it harder more often (not always possible)
Buy a gas tractor for light work (expense of two)
Trade it for a Gas or LPG tractor and be done with it.
Live wit hit. Jim

"Wet stacking" is not uncommon for a diesel when it is not being worked hard. As to why it didn't do it before I cannot say. You have gotten some good advice here. If it has been a long time since any tune up was done, have the valves set and pull the injectors and have them tested at a minimum. After that use top grade fuel and maybe even consider a conditioner of some kind. I don't think you have a serious problem but it might be an indication of something that is needing attention and will cause problems down the road. Take good care of those D407's and they will take good care of you.