Ford KA pulling when accelerating and stalling, also black smoke out exhaust

hi my ka since having the tracking and oil topped up has been 'pulling' or losing power when driving, when slowing and stopping at junctions and oights it stalls, my brother noticed black smoke come out the exhaust when i started it up. recently i had a knock to the left/passenger side light/bumper it was nothing major but cracked bumper and pushed light in slightly. i also had tracking done and they topped my oil up. any one have any ideas what could be wrong and what may need to be repaired? thanks