This is the CJ-3B hoopla. The Hurricane F-head engine was an upgrade of the Go-Devil L-head, and they had many interchangeable parts. In fact, you could make an L-head into an F-head and there was once a kit available to do this. The exhaust valve remained in the block, but the cam profile differs between the two engines. A high-compression head (7.4:1) was available for high altitude, and at sea level, those engines produced 75 hp and 119 lb-ft. These heads were often painted yellow from the factory.