Smoke and flame was observed issuing from the engine exhaust area during start up from a medivac operation. The pilot shut down, broadcast a Mayday and evacuated the patient and crew. The subsequent investigation disclosed that one engine had not lit off properly and the pilot had not accomplished the correct blow out procedure prior to attempting another start. When the engine lit off on the next start some excess fuel had overheated and ignited.This is the normal result following an over rich start and inadequate blowout. This occurred at night at a road accident location which was surrounded by lights and flashing beacons, which caused ground based personnel to conclude that the helicopter was on fire. Examination of the helicopter showed that no damage had occurred. Significant factors: The following factors were considered relevant to the development of the incident. 1. The pilot failed to carry out the correct blow out procedure after one engine failed to start. 2. During the subsequent start attempt residual fuel ignited generating smoke and flame.