On June 25, 2018, about 0830 eastern daylight time, an experimental amateur-built Sibila MH1 helicopter, N32KS, impacted terrain during a forced landing near Beach City, Ohio. The pilot received serious injuries; and the helicopter sustained substantial damage. The helicopter was owned and operated by an individual under the provisions of Title 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed at the time of the flight, which was not on a flight plan. The helicopter departed about 0825 from Beach City Airport (2D7), Beach City, Ohio, on a local flight. The pilot reported that he departed on a maintenance flight to track and balance the helicopter's main rotor. He made several flights to make adjustments to the main rotor. On the last flight, he was making the approach to land and he was about 40 ft above ground level (agl) when the engine "completely shut down" without any warning. He entered an autorotation for the forced landing, but within seconds the helicopter impacted the terrain, the skids collapsed, and it rolled on its side. The owner and builder of the helicopter witnessed the accident and was located about 3,000 ft from the accident site. He stated the helicopter appeared to be about 20 to 30 ft agl and he thought the pilot planned to turn toward the runway. Then the helicopter appeared to enter a steady descent until ground contact. When he arrived at the accident site, the helicopter pilot told him that the engine just "quit." The examination of the helicopter revealed that all three carburetors were dislodged from the intake manifold. The outer two clamps of the intake manifold appeared to be loose, and there were no threads protruding through the outboard clamps. The third intake manifold clamp was found at the initial impact area, and the clamp appeared to be tight in reference to the threads protruding through the clamp nut. The engine was a 3-cylinder Yamaha Genesis 120-horsepower engine that was originally designed to be used in snowmobiles. It was a 3 carburetor 4-stroke engine with slaved carburetors with dual electrical fuel pumps. It was a rebuilt zero-time engine and it had 9.1 hours on the hour meter at the time of the accident. The engine was prepared for an engine run/test, including re-installing the intake manifold and tightening all the clamps. Full throttle travel was checked, and the fuel pumps were turned on and it was confirmed that there were no fuel leaks. The engine started with no hesitation, but the run time was limited due to damaged cooling fans. After the second engine run, the outboard intake manifold clamps were loosened to the point were no threads protruded through the clamp, but the center clamp was left in the tightened position. The carburetor assembly was forcefully pushed and pulled but the induction system did not come loose from the intake manifold. The pilot reported that he held a private pilot's certificate with single-engine, multi-engine, and airplane instrument ratings. He did not hold a rotorcraft rating. He had 3,200 total flight hours with 260 hours in the make and model of the accident helicopter. He stated that he received helicopter training in a Bell 47 and had received autorotation training and had practiced autorototions and sliding landings. The pilot reported that as a distributor for the experimental, amateur built helicopter, he typically flew the maintenance flights to track and balance the main rotor system of the new amateur-built helicopters.