As the pilot was applying power at the commencement of the take-off run, the right engine lost power. An explosion was then heard from the vicinity of the right engine. The pilot secured the engine and after stopping the aircraft, evacuated the passengers before leaving the aircraft himself. He then noticed a small fire under the right wing which he quelled with a fire extinguisher obtained from the aircraft. Investigation revealed a loose fuel line union in the right wing just outboard of the engine. This allowed fuel to leak into the wing where it pooled and the vapour was ignited by the hot exhaust gases. No record was found of any maintenance being carried out on the fuel line in question.