Small Engine Troubleshooting Problems? Ask a Mechanic Now.
Have you looked at the valve action? Does spark plug show any damage? If valves check good, and no damage to the spark plug. And I assume the engine will not turn with spark plug removed? I would think problem may be with the pump. To tell for sure, you are going to have to get the pump off the engine, or disassemble it on the engine.
You could remove the cylinder head!
Or remove the timing chain.