THE ACFT LANDING GEAR COLLAPSED DURING AN OFF RWY BUT ON ARPT FORCED LANDING. THE PILOT HAD JUST COMPLETED THE ANNUAL INSPECTION AND WAS TESTING THE ACFT. AT 150 FT AGL DURING TAKEOFF THE ENGINE QUIT. THE PLT LANDED ON ROUGH TERRAIN ABOUT1/4 MILE WEST OF THE DEPARTURE END OF THE RWY. THE POST ACCIDENT INVESTIGATION REVEALED MINOR FOREIGN CONTAMINATES IN BOTH FUEL TANKS. THE MAIN FUEL LINE TO THE CARBURETOR INLET UNION DID NOT CONTAIN ANY FUEL. FUEL HOWEVER WAS FOUND IN THE GASCOLATOR BOWL. THE ELECTRIC FUEL BOOST PUMP FILTER CONTAINED EXTENSIVE FOREIGN CONTAMINATES. THE CARBURETOR AND FUEL PUMP WERE BENCH TESTED AND THE CARBURETOR BOWL FLOODED AT AN INPUT PRESSURE OF 3.8 # PSI. AT 5# PSI THE FUEL LEVEL IN THE BOWL SHOULD BE ABOUT 1/2 INCH FROM THE BOWL TOP WHEN THE FLOAT SHUTS OFF THE FUEL. THE FLOAT ASSEMBLY DID NOT SUBMERGE WHEN PLACED IN FUEL. THE FLOAT LEVEL WAS SET AT 8/32 INCH. THE SERVICE LIMIT IS 7/32 INCH. ACCORDING TO A TESTING TECHNICIAN THE FAILURE OF THE FLOAT WAS PROBABLY DUE TO CONTAMINATION IN THE FLOAT VALVE SEAT.