Does the engine shut down with the key? If so then it's fuel pump related. If not, it's oil related. My guess is fuel pump. Did it do this before your rebuild?
anyway if you are capable of taking the pump apart try that and see if the govenor is sticking. thats all I can think of thats wrong.
P.S. does anyone have a link to the post explaining deo's meltdown. i tryed seaching for it but couldnt find it. thanks alot mark
it could be that the pump timming is advanced too far, that will make black smoke and can dammage your engine if its left like that so before anything get the pump timming done