Hi guys 

Recently viewed a cat D2 with the potential of buying, tractor has been overhauled and main engine rebuilt, on arrival it was running but the exhaust emissions at idle were quite Smokey, found heavy slobbering from the stack and manifold with oil running down the block, during testing it seem to be mis- firing and fuel was leaking from the leak off pipe, been assured that the injectors have not been done during overhaul. Would anyone agree that this tractor needs a good heavy load to clear this slobbering after the injector have been repaired? 

yes warm it up and work it not hard but make it work.

Make it work

Since you are looking to buy, your kind of stuck. But if it was mine make sure all the water and stuff is drained out the drain cock on the tank. Make sure that corner is low. Change the fuel filters, then run it hard.

Not knowing the seller and bargaining already done? That is most of a day to see if that is all it takes. Does the price go up, or it has proved it needs more parts.

That leak off line is there for a purpose, it will leak. But the question is how much is to much.

Slobbering is a good sign it has been babied, been setting awhile or the regulator (thermostat) is opening too soon and can not come up to operating temp. A standard 175*F thermostat starts to open at ~166*F and mid open at ~175*F and wide open at 188*F ....
running temp .... Anything lower than a continuous ~168*F will cause the exhaust stack to start slobbering - not hot enough to keep condensation from forming. Run the puppy water out of it to get the temp on step but keep your eye on the gauge. Been babied or run underpowered loads up the engine oil, head and exhaust with carbon. If it has been setting for a long time the cylinder walls dry out a bit and the rings will need to reseat ... puppy water extraction helps with all these issues.
Once the engine stops slobbering the exhaust may take several hours to clear. Gaskets, including the head gasket, may begin to weep and leak. Clean and keep a eye out and again after the heat comes up the gaskets may begin to re seal too. It wants to run.
Other considerations if that don't help are make sure the injectors have a good spray, valve lash and injection timing are in spec.

put some injector cleaner in it , and hook it to a disk or something and the crap out it, and it will cleanup and then worry about any issues then

you could can think about installing a 195F thermostat in to it as that will help get and keep the temps up as it hard to get the under full load for a long time to get heat in to them. unless it was broke in on a dyno i would say the new ring never got seated and that's why the the sobering.