Paul, I suggest you check the cam belt timing because it is easy to do. Even if the belt has jumped, retiming it is easy to do too. The only hard thing is if the belt has deteriorated, because you can't replace it without splitting the engine's crankcase, and that is a middle-sized hassle on the GCV engines because they have non-detachable cylinder heads, so the crankcases split diagonally to enable the pistons to be removed from underneath.

Remove the camshaft and rocker cover, set the crankshaft accurately on TDC on the compression stroke, and look at the timing marks on the plastic camshaft/sprocket as shown in this illustration from the public access service manual:

[Linked Image]

If the timing marks are both visible and level with each other when the crankshaft is in that position, the cam timing is correct. If they aren't level, say so and I'll explain how to fix it - a very simple operation. However if the belt shows signs of wear, life will get a bit harder.