THE PLT & 3 PSGRS WERE ON A X-COUNTRY SIGHTSEEING FLT, WHEN THE ENG BGN TO RUN ROUGH, THEN LOST PWR. SUBSEQUENTLY, THE ACFT COLLIDED WITH TRRN DRG A FORCED LANDING & HIT A TREE BFR IT CAME TO REST. WHEN CHECKED AFTER THE ACDNT, THE ENG WOULD RUN, BUT ONLY AT IDLE. THE ENG WAS EQUIPPED WITH A MARVEL-SCHEBLER MODEL HA-6 CARBURETOR (CARB). AS THE CARB WAS BEING REMOVED FM THE ENG FOR EXAMINATION, THE ROTARY MIXTURE CONTROL (PN 242-541) FELL OUT OF THE CARB HOUSING BECAUSE THE SHAFT RETAINING SCREW HAD BKN. IN SEP 78, MARVEL-SCHEBLER/TILLOTSON (ORIGINAL MANUFACTURER OF THE CARB) ISSUED SVC BULLETIN (SB) #A1-78, BUT IT HAD NOT BEEN ACCOMPLISHED. THE SB PROVIDES FOR INSTLN OF A SPRING RETAINER (PN 55-A239) TO COUNTERACT END-PLAY WEAR ON THE MIXTURE CONTROL ASSEMBLY & RETAINING SCREW DUE TO VIBRATION. THE SPRING RETAINER ALSO SERVES AS AN ADNL MEANS OF RETAINING THE ROTARY MIXTURE CONTROL, EVEN IF THE EXISTING SHAFT RETAINING SCREW IS WORN OR BKN. THE ENG HAD A TOTAL OPERG TIME OF 2568 HRS, 485 HRS SINCE OVERHAUL & 4.7 HRS SINCE THE PREVIOUS 100 HR INSPN.