Have you ever carried out a 'de-coke'? Chances are the valves are pretty much stuck with carbon build up.

You need to remove the exhausts and look up the header into the engine. If it's all black and sooty, and crud up with soot, you'll need to get it all cleaned off.

This involves removing the shaft from the side of the exhaust port (where the valve drive pulley bolts on) and dropping the valves out through the exhaust.
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