SEVERAL DAYS PRIOR TO THE ACCIDENT, THE PLT WAS ADVISED BY OTHERS THAT THE ENG ON HIS NEWLY ASSEMBLED ULTRALIGHT DID NOT'SOUND RIGHT.' AFTER CHECKING HE NOTED THAT FULL POWER COULD NOT BE OBTAINED FROM THE ENG. HOWEVER, ENOUGH POWER WAS AVAILABLE TO FLY A NUMBER OF LOCAL FLTS. HE ALSO NOTED BLACK SOOTY DEPOSITS ON THE SPARK PLUGS & THAT THE ENG AIR INTAKEFILTER WAS SOAKED WITH FUEL. THIS FLT WAS TO FURTHER TROUBLESHOOT THE ENG PROBLEM. THE ENG LOST POWER AT 200 FT AGL & THE PLT DECIDED TO RETURN TO THE ARPT. THE PLT OVERFLEW AN OPEN FIELD & THE VEHICLE STALLED AVOIDING RISING TERRAIN & TREES. THE PLUNGER OF THE STARTING MIXTURE ENRICHMENT SYSTEM/FUNCTIONS AS A CHOKE) WAS FOUND BOUND UP AS IT PASSED THROUGH THE CARB HOUSING WHILE MOVING TO THE 'DESENGAGED' POSITION. THE ACTUATOR CABLE HOUSING FLEXED UNDERNEATH THE FIBERGLASS NOSE FAIRING WHEN THE PLUNGER STOPPED MOVING, ALLOWING THE KNOB TO RETURN CLOSE TO ITS NORMAL 'CHOKE-OFF' POSITION.