RedactedNarrative: V1 was being operated by O1 on Shadehill Reservoir.  V1's motor quit and it was taken to shore where it was beached.  O1 and P1 were working getting V1's motor running again.  O1 attempted to start V1 and a small explosion occurred and V1 caught on fire.  P1 was working on the motor when the explosion occurred.  O1 and P1 exited the boat and a nearby PWC operator used his wake and wash and put the fire out.  No injuries were reported to WCO at the time of the report.