If so I would take it back to your mechanic and have them adjust it. If yours aren't adjustable check with the dealership to see what size injectors your intake requires and check that against what you have now. It's possible that the replacement injector is too small and causing that cylinder to run lean. Check for a part number on the injector and ask the dealership to verify that it is the correctly sized injector or equivalant. If your engine is computer controlled, a tune-up may help as well.
Some fuel injectors are adjustable.
Try calling a dealership and ask if this is the case with yours. If so I would take it back to your mechanic and have them adjust it. If yours aren't adjustable check with the dealership to see what size injectors your intake requires and check that against what you have now. It's possible that the replacement injector is too small and causing that cylinder to run lean. Check for a part number on the injector and ask the dealership to verify that it is the correctly sized injector or equivalant.