A TEST PLT & DEALER FOR THE MANUFACTURER INSPECTED THE ULTRALIGHT VEHICLE & MADE ADJUSTMENTS PRIOR TO MAKING A TEST FLIGHT AFTER IT WAS ASSEMBLED. HE STATED THAT BEFORE TAKEOFF, HE WAS HAVING PROBLEMS SETTING THE CARBURETOR MIXTURE & THAT THE THROTTLE CABLE WAS NOT WORKING WELL. DURING EARLIER TAXI TESTS, THE ENG CEASED OPERATING A NUMBER OF TIMES. FINALLY, ON THE LAST TAXI TEST, THE ENG OPERATED AT FULL RPM & HE ELECTED TO TAKEOFF. AFTER TAKEOFF, THE ULTRALIGHT CLIMBED TO ABOUT 100 FT AGL, THEN BEGAN TO LOSE POWER. THE PLT WAS UNABLE TO GLIDE TO A CLEARING & SUBSEQUENTLY CRASHED INTO TREES. THE VEHICLE REMAINED IN THE TREES & THE PLT REMAINED SECURE BY HIS LAP BELT & SHOULDER HARNESS. HOWEVER, AFTER HE RELEASED HIS RESTRAINT SYSTEM, HE WAS INJURED WHEN HE FELL APRX 40 FT TO THE GROUND. THE TEMP & DEW POINT WERE 40 & 36 DEG, RESPECTIVELY. ACCORDING TO ICING PROBABILITY CURVES, SERIOUS ICING WOULD HAVE BEEN POSSIBLE AT CRUISE POWER. HOWEVER, ICING WAS NOT VERIFIED. THERE WAS NO PROVISION FOR CARBURETOR HEAT.