A Westjet Boeing 737-700, registration C-FIWJ performing flight WS-555 from Calgary,AB to Vancouver,BC (Canada) with 69 people on board, had just levelled off at cruise level 400 about 90nm westsouthwest of Calgary when the crew declared emergency reporting fumes in the cockpit and decided to return to Calgary, where the aircraft landed safely about 17 minutes later. The aircraft was examined by emergency services and taxied to the apron on own power. The Canadian TSB reported the aircraft was climbing through 7000 feet out of Calgary when a burning odour in the cabin occurred. The crew subsequently declared emergency and returned to Calgary. Maintenance inspected the aircraft and found no faults. Checking the technical log it was found the #2 engine had been replaced immediately prior to the flight. A high power run was performed on both engines with no smell detected. Bleed valves and ducts were checked for oil contamination, none was found. A verification flight was completed with no odour. The occurrence aircraft returned to service 21 hours after landing back into Calgary.