Just the simple fact that fuel lines were disconnected and reconnected means that something got disturbed or dirt was possibly introduced. Even tilting the engine on R & R could cause some dirt to move from a place where it didn't cause problems to a place where it now IS a problem.
Check your plugs and wires. Scratch that, check the entire ignition system. Tug on things, check your connection points. Chances are it is something simple but easily overlooked.