WHILE TAKING OFF, THE ACFT CLIMBED APRX 100 TO 200 FT, THEN BEGAN LOOSING ALT. DURING A FORCED LANDING ON A STREET, IT HIT A PARKED CAR & A LIGHT POLE & BURST IN FLAMES. THE PLT, WHO LATER EXPIRED, SAID THAT NEITHER ENG DEVELOPED FULL POWER (2575 RPM). HE SAID THIS WAS NOT UNUSUAL WHEN POWER WAS 1ST APPLIED, BUT SAID FULL RPM WAS GENERALLY OBTAINED SHORTLY AFTER THE ROLL WAS BEGUN. AFTER BEING COMMITTED FOR TAKEOFF, BOTH RPM'S WERE REPORTEDLY BETWEEN 2300 & 2400 RPM.SVRL WITNESSES STATED THE ENGS SOUNDED LIKE THEY WERE RUNNING ROUGH. A WITNESS REPORTED THE ACFT SWERVED 'AS THO THE PLTHAD DIFFICULTY WITH THE L ENG.' REPORTEDLY, THE PLT 'DROPPED SOME MORE FLAPS' BELOW THE 20 DEG TAKEOFF SETTING 'IN ORDERTO PULL IT OFF' & RAISED THE GEAR IMMEDIATELY AFTER LIFT-OFF. ENG TEARDOWNS REVEALED EXCESSIVE SPARK PLUG GAPS, 1 MAG ONTHE R ENG HAD A .008 POINT GAP (.016 REQUIRED), ITS #1 & #4 DISTRIBUTOR BLOCKS WERE CRACKED & THE #1 & #6 LEADS WOULD NOT FIRE PROPERLY. HARTZELL F-6-5A PROP GOVERNORS INSTALLED IN PLACE OF F-6-35A. ACFT WAS APRX 348 LBS OVER MAX WT LMT.