WITNESSES REPORTED THAT DURING TAKEOFF, THE ENG BEGAN SPUTTERING WHEN THE ACFT WAS CLIMBING THRU ABOUT 50 FT AGL, THEN THE ENG NOISE STOPPED COMPLETELY AS THE ACFT REACHED ABOUT 150 FT AGL. A PUFF OF SMOKE WAS OBSERVED COMING FROM THE ENG AS THE ACFT VEERED TO THE RIGHT & ENTERED A STEEP RIGHT BANK. THE ACFT THEN LOST ALTITUDE RAPIDLY & CRASHED ABOUT 150 FT SOUTH OF THE DEPARTURE END OF RWY 9. AN INVESTIGATION REVEALED THAT THE LEFT FRONT CYLINDER FUEL INJECTOR NOZZLE FLOW WAS ABOUT 40% LESS THAN THE OTHER 3 NOZZLES. A PIECE OF RUBBER TYPE MATERIAL WAS FOUND RESTRICTING THE FUEL FLOW TO THAT NOZZLE. THE MATERIAL WAS IDENTIFIED AS BUTADIENE/ACYLONITRILE, WHICH IS THE SAME MATERIAL THAT WAS USED IN SOME FLEXIBLE FUEL LINES. BOTH PLTS RECEIVED SERIOUS HEAD INJURIES. THE SURVIVING PLT IN THE BACK SEAT COULD NOT REMEMBER THE FLT OR ACCIDENT. SHOULDER HARNESSES WERE AVAILABLE & USED, BUT NEITHER WERE EQUIPPED WITH INERTIA REELS. FLT MANUAL & 14 CFR 23.785 STATES SHOULDER HARNESSES SHOULD BE LOOSE ENOUGH FOR FLT OPS.