HISTORY OF FLIGHT: On August 28, 1994, at 1410 central daylight time, a Cessna 402, N966JW, was substantially damaged during a forced landing near Harlingen, Texas. The commercial pilot and six passengers received minor injuries. The Title 14 CFR Part 135 on demand operator was Airways Adventures of Harlingen, Texas. Visual meteorological conditions prevailed for the non-scheduled passenger flight to Matamoros, Mexico. During a telephone interview the pilot reported the following information. He added full power for the takeoff from runway 17L and the gear was retracted following liftoff. Seconds after retracting the gear the "vertical velocity and forward velocity were decreasing" as a partial loss of power occurred. From 150 feet to 200 feet MSL, a forced landing with gear up was made to a plowed field beyond the departure end of the runway. Federal Aviation Administration (FAA) inspectors reported the information in this paragraph. The airplane went through customs at Brownsville, Texas, that morning and then landed at San Benito, Texas, where the airplane was refueled. Throughout several attempts by the pilot and the operator, the left engine would not start. A mechanic who was contacted to start the engine removed the engine magneto "P" leads and started the left engine. The mechanic told the operator and the pilot that there was no way to check the operation of the magnetos with the "P" leads removed. The airplane was flown to Harlingen, Texas, to load the passengers destined for Matamoros, Mexico. At Harlingen, Texas, the right engine would not start and the mechanic removed the "P" leads from the right engine. Passengers reported the information contained in this paragraph. Six written statements are enclosed. The arrival of the airplane was delayed due to mechanical problems and at Harlingen a mechanic was called to assist in getting the right engine started. One passenger reported the baggage was "crammed into the luggage compartments." Subsequently, the pilot taxied the airplane to the end of the runway and was cleared for takeoff. Neither run-up checks nor passengers briefings were done. The pilot continued the takeoff roll; however, the airplane was not accelerating. The airplane finally lifted off the runway and gained altitude as the gear was retracted. Three of the passengers reported the stall warning right after takeoff. After the accident, the pilot left the scene, and Gulf Aviation personnel immediately unloaded the luggage. Passengers were assisted by the airport fire and rescue personnel. AIRCRAFT INFORMATION: Maintenance records indicated that engine S/N 237010R was installed in the left hand position of N966JW on May 9, 1994, with a TSMOH as 828.8 hours and the turbocharger was changed on June 21, 1994. The installation date of engine S/N 183244R in the right hand position could not be determined by the available records. Weight and balance data (enclosed copy) calibrated in May, 1993, indicates a maximum allowable gross weight (enclosed) for takeoff as 6,300 pounds which includes a basic empty weight of 4,087 pounds and a useful load of 2,213 pounds. The flight load manifest (enclosed copy) signed by the pilot indicates a total aircraft weight of 6,281 pounds for this flight. On the enclosed reports, the owner, operator, and pilot stated 120 gallons of fuel on board at takeoff. Thus at 6 pounds per gallon, the total fuel weight was 720 pounds. Passenger statements (enclosed) indicated a total passenger/baggage weight of 1,663 pounds. FAA medical records state the pilot's weight as 177 pounds. Summation of the basic empty weight, oil (49 pounds), pilot, passenger/baggage weight and fuel weight gives a total takeoff weight of 6,696 pounds. This is 396 pounds over the maximum allowable gross takeoff weight. With a takeoff temperature of 95 degrees Fahrenheit, the density altitude was 2,600 feet. WRECKAGE AND IMPACT INFORMATION: FAA inspectors examined the airplane and reported that the fuselage and right wing sustained substantial damage. Flight control continuity was established and fuel was found in the tanks. They confirmed that the "P" leads for all magnetos had been removed. TEST AND RESEARCH: The airplane was examined on September 20, 1994, at Air Salvage of Dallas, Lancaster, Texas. Continuity was confirmed for each engine. Differential compression checks on the engines found 4 cylinders below 60 PSI for the left engine and 5 cylinders below 60 PSI for the right engine. Points on all the magnetos were burned and the left magneto on each engine would not fire. All spark plugs exhibited wear and deposits in the electrode areas. Bottom plugs from the left engine cylinders #3 and #5 would not fire and the bottom plugs from the right engine cylinders #1, #3, #4, #5, and #6 would not fire. On the left engine, the fuel unit throttle body was not clamped at the turbocharger. See the enclosed engine report for additional details. ADDITIONAL INFORMATION: The airplane was released to the owner's representative.