Can you clarify the problem with the exhaust valve Rob? What had come loose? Engines sometimes "drop valves", meaning the valve stem cotter falls off, but that usually results in a smashed engine. The valve seat insert might come loose, but that would not prevent the engine from turning over normally. Sometimes with overheating, the exhaust valve guide comes loose and slides upward, stopping the engine from rotating before the exhaust valve can open fully - maybe that's a possibility, though I don't see how a jammed exhaust valve would cause it to pop through the carburetor, especially since it would happen during the exhaust stroke, nowhere near the firing position.
