THE PURPOSE OF THE FLT WAS TO DEMONSTRATE THE ACFT TO A POSSIBLE BUYER. WHILE TAKING OFF, THE ENG LOST POWER SHORTLY AFTER THE ACFT LIFTED OFF AT ABOUT MID-FIELD. THE PLT ATTEMPTED TO TURN THE ACFT & LAND ON A CROSS-RUNWAY. HOWEVER, DUE TO HIS LOW SPEED & LOW ALT, HE WAS UNABLE TO ALIGN THE ACFT WITH THE RWY. THE ACFT WAS LANDED IN GRASS WHERE IT SUBSEQUENTLY NOSED OVER. THE ENG HAD ONLY 1 HR OF FLT TIME SINCE IT HAD BEEN OVERHAULED. DURING THE OVERHAUL, A FACET AEROSPACE CARBURETOR FLOAT KIT, NUMBER 666-915, HAD BEEN INSTALLED IN THE MARVEL SCHEBLER CARBURETOR BY THE PLT/OWNER, WHO WAS ALSO AN A&P MECHANIC. AN EXAM REVEALED THAT SERVICE BULLETIN NUMBER A1-84A WAS NOT COMPLIED WITH, THE FLOAT DROP WAS NOT ADJUSTED PROPERLY, FUEL FLOW WAS LOW, THE THROTTLE SHAFT WAS LOOSE, THE ACCELERATOR PUMP WAS DRY ROTTED & CRACKED, & THE BUTTERFLY VALVE WAS INSTALLED UPSIDE DOWN.