JUST AFTER TAKEOFF, SHORTLY AFTER LANDING GEAR RETRACTION, THE ENG BACKFIRED AND EXPERIENCED A TOTAL POWER LOSS. THE PLT LOWERED THE LANDING GEAR AND LANDED STRAIGHT AHEAD JUST OFF THE RWY. THE ACFT BOUNCED ONCE AND CAME TO REST IN A CITY PARK. IT IMMEDIATELY CAUGHT FIRE. THE TWO OCCUPANTS ESCAPED FROM THE ACFT, BUT WERE SERIOUSLY BURNED. INSPECTION OF THE ENGINE FOLLOWING THE ACCIDENT SHOWED EVIDENCE OF OVERHEATING. THERE WERE INDICATIONS ON ALL SPARK PLUGS OF HEAVY DEPOSITS OF LEAD AND OIL. THE SPARK PLUGS WERE NOT GAPPED CONSISTANTLY. THE ELECTRODES WERE GRAY IN COLOR. TESTS CONDUCTED ON THE PLUGS REVEALED THAT THE MAJORITY FAILED TO FIRE AT HIGH MANIFOLD PRESSURES. THE DENSITY ALTITUDE WAS APPROXIMATELY 10000 FT MSL. THE PLT STATED THAT ON RUN-UP THE ENGINE RAN ROUGH, BUT LEANING THE MIXTURE HELPED. HE STATED THAT HE ADJUSTED THE MIXTURE TO FULL RICH FOR TAKEOFF.