On August 20, 1999, at 1510 hours Pacific daylight time, a Mooney M20E, N6022Q, was substantially damaged when it impacted terrain during a post maintenance check flight at Lampson Airport, Lakeport, California. Visual meteorological conditions prevailed and no flight plan was filed. The aircraft, operated by Lake Aero Styling & Repair under the provisions of 14 CFR Part 91, was on departure climb out from Lampson at the time of the accident. The commercial licensed pilot was seriously injured. The post maintenance check flight was performed following repair of damage caused by a gear-up landing. According to the operator, the repairs consisted of replacement of (fuselage) belly skin panels and nose gear doors, replacement of the propeller, and removal and reinstallation of the engine. The engine was shipped to an engine repair station for a "prop strike" inspection. An annual inspection of the aircraft was performed coincident with the repairs. The engine repair station reported that the engine was disassembled and inspected for a prop strike in accordance with applicable Textron Lycoming Service Bulletins. No damage from the prop strike was found. Although the engine was received as an assembly with the engine mount and baffling installed, the engine repair station returned the assembled engine with the engine mount and baffling not installed. A witness reported the aircraft's engine sounded normal during takeoff on runway 28 (3,600 feet long, 60 feet wide), and he observed the landing gear retract during initial climb out. He then observed the landing gear extend again and it appeared to the witness that the pilot was attempting to reverse course to land on the departure runway. During the turn, the aircraft stalled and crashed in a vineyard. The pilot reported his recollection that soon after takeoff, while passing the end of the runway, the engine started losing power, "running rough". He started a left turn, intending to land downwind on runway 10 using the partial engine power available. During the turn the engine stopped completely and because of low airspeed and altitude, the pilot was unable to glide the aircraft to the runway. An inspector from the Federal Aviation Administration (FAA) Sacramento FSDO examined the engine and reported that the spark plugs were clean and light gray color, and the oil and oil filter were clean. There was clean fuel in lines and fuel tanks. When the fuel lines were examined it was found that there was a loose B-nut on the line from the fuel control servo where it attaches to the fuel divider on top of the engine. When the electric fuel pump was turned on fuel sprayed from the loose nut. The inspector reported that the B-nut required about one turn in the tightening direction to reach the snug position. The inspector noted that there was "torque seal" present on the threads of the elbow fitting but not on the body of the B-nut. No other discrepancies were noted. On October 4, 1999, the engine was installed on a test stand at Clarksburg Air Repair, Clarksburg, California. The muffler was removed due to impact damage; however, the flame tubes were intact. A cracked casting in the intake manifold was repaired using soft putty. With the above mention B-nut tight, the engine was started and ran smoothly. A magneto check at 1,700 rpm produced drops of 75 - 100 rpm's on each magneto. The engine was accelerated to full power and smoothly produced about 2,700 rpm using a fixed pitch test propeller (test club). After about 5 minutes at full throttle, the above mentioned B-nut was loosened. After 1/8 turn fuel came out of the hose end. After 1/4 turn the engine began to run rough and between 1/4 and 1/2 turn the engine abruptly stopped. The B-nut was again tightened and the engine was started. The engine was operated at full throttle again for 3 minutes and produced smooth power about 2,700 rpm. Another additional person (party) was Mr. Danny S. Phillips, Lake Aero Styling & Repair, Lakeport, California 95453.