Engine Sputtering after 351W swap(EFI)

[FritzDaKat]

Well if the block had been recently rebuilt, you are supposed to put about 300-500 miles on it before driving it like a race car. if you had driven it hard without it wearing in, you could be leaking oil thru the valves, past the piston ring's ect. which would account for all the black smoke.

If you havent been driving it like a madman, I'd just take it easy on the engine for a few days, also make the rebuilders aware of whats going on, mabey they have some thought's or suggestions on it, and just see if it starts clearing up any.