On March 14, 2004, about 1800 Pacific standard time, a Quinn Aviation Inc., E L Tom Cat MK-5A, N8429, made a forced landing following a total loss of engine power while cruising between fields near Galt, California. Sierra Aviation, Inc., was operating the helicopter under the provisions of 14 CFR Part 137. The commercial pilot, the sole occupant, was not injured; the helicopter sustained substantial damage. The local aerial application flight departed a farm field near Galt about 1750. Visual meteorological conditions prevailed; no flight plan had been filed. The primary wreckage was at 38 degrees 18 minutes north latitude and 121 degrees 18 minutes west longitude. The operator submitted a Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2). The pilot finished spraying an alfalfa field, and headed toward the next field that was 1/4 mile away. The helicopter shook violently for 2-3 seconds followed by a loss of engine power. The pilot performed an autorotation from 100 feet, and the helicopter sustained substantial damage. The operator examined the helicopter and engine and reported that he drained water from the fuel tank; he found a small amount of water in the bottom of the fuel filter screen; the screen was dirty but not clogged; and he found no water when he drained the carburetor. The engine turned freely and all of the cylinders developed compression between 72/80 and 78/80. The operator noted no other abnormalities that would explain the loss of engine power.