mini trouble

I took my 1992 mini city out 4 a drive 2day and she started up fine first
time, drove 4 about 30min and then parked up. Got back in the car and it
wudnt start, the starter motor was working but the engine wudnt fire up, i
checked the starter leads and they seemed ok, i had a friend with me so we
decided to try and jump, but no joy. After trying to start for about five
minutes she suddenly started and a big black smoke came out the exhaust,
with a few bangs. Fine i thought and put it down to a fuel block, the car
seemed fine until i came to some traffic lights and the revs started to
fall and i managed to catch it before it stalled, however at the next stop
i didnt and the car stalled-just as before the engine wudnt fire, we moved
the car to the side of the road and came back to it about 20mins later,
its started first time and the revs were dropping a bit but not as bad as
before! Im thinking that it cud been  a spark plug or timing problems! Any
advice wud be helpfull!!!!!!!