When i dropped my engine in my cabby it started and ran fine, but back fired something chronic, i just assumed it was cos it wasnt set up, took it to a mate to set it up and it was chucking out something ridiculous like 1500+ hydrocarbons out the exhuast, then the next minute it was going right down. We found 1 cylinder to be overfuelling by the plugs, 1 being totally black, metering head was fucked. Have a look at your plugs they will tell you a lot...