1958 Hercules engine with valve problem. I couldn't get either valve to work on #2 cylinder. Finally removed the manifold and have pulled 31 acorns from the exhaust holes in the manifold and exhaust valve holes in the block. There are more that I can see down the exhaust pipe. Boat has been stored in the previous owners barn for 30+ years. I noticed a pin across the exhaust pipe when it was detached from the manifold. Should there be a "butterfly" check valve there to prevent stuff from entering the exhaust manifold?