The engine lost power shortly after liftoff and the pilot made a forced landing in a gravel pit, intentionally stalling the airplane to obtain the lowest possible landing speed. The airplane had a STC for and was being operated on automotive gasoline. Residual fuel and debris were noted in the main fuel tank. The filter screen was estimated to be 20% blocked by sand and debris. Some water was found in the fuel. The carburetor needle valve was found to be binding. It had a flat spot on one side, allowing fuel to flood the bowl. The #1 spark plug was inoperative. The #3 plug was oil soaked. All plugs and the exhaust pipe had carbon deposits. Two days before the accident, an airport worker started the engine and it ran for a short period of time before it stopped. No reason was given for the stoppage, and there was no evidence that it was investigated further. The airplane was not equipped with shoulder harnesses.