While flying downwind at 30 knots about 50 feet above the ground, the pilot attempted to increase power to accelerate the aircraft. There was no response from the engine, and engine and rotor RPM fell as he applied collective pitch. He attempted to slow the aircraft to land but the heel of the left skid hit the ground and the helicopter rolled on to its side. Examination of the helicopter revealed that a very close tolerance fitting in the linkage between the throttle valve and the mixture control valve caused severe binding. This prevented normal throttle movement.
