DURING ARRIVAL, THE PLT REDUCED POWER ON BASE LEG FOR FOR A LANDING. WHEN HE REALIZED HIS AIRSPEED WAS DISSIPATING FASTER THAN NORMAL, HE ADVANCED THE THROTTLE, BUT THE ENG DID NOT RESPOND. THE PLT THEN SWITCHED FUEL TANKS, TURNED ON THE BOOST PUMP & APPLIED FULL THROTTLE, BUT THE ENG STILL DID NOT RESPOND. SUBSEQUENTLY, WHILE FLARING FOR A FORCED LANDING, THE ACFT HIT A TREE & CRASHED. BOTH OCCUPANTS WERE WEARING THEIR SHOULDER HARNESSES & WERE NOT INJURED. AFTER THE ACCIDENT, THE FUEL PRIMER WAS FOUND UNLOCKED & EXTENDED ABOUT 1-1/2 INCHES. DURING AN OPERATIONAL CHECK, THE ENG STARTED & RAN NORMALLY AT 1500 RPM. WHEN THE PRIMER WAS UNLOCKED & PARTIALLY OPENED, THE ENG BEGAN RUNNING ROUGH, ITS SPEED DECREASED TO 700 RPM & THE ENG EMITTED BLACK SMOKE THRU THE EXHAUST. THE MANUFACTURER CONFIRMED THAT AT LOW RPM WITH THE PRIMER OPEN, FUEL WOULD SIPHON INTO THE MANIFOLD & CAUSE THE ENG TO RUN TOO RICH. ORIGINALLY, THE PRIMER HAD BEEN INSTALLED AS OPTIONAL EQUIPMENT FOR STARTS IN EXTREMELY COLD WX & WAS NOT LISTED IN THE FLT MANUAL CHECK LIST.