Paul, is it cycling in speed continuously, with a cycle period of about 5 seconds? This is called hunting, and is usually due to lean mixture, but it can occasionally be due to binding in the governor linkage. On the other hand if the engine speed simply varies widely and erratically, it is not hunting and may be due to various causes.
Because your engine is a GCV, it has a toothed-belt driven overhead camshaft, not the pushrod system used on the commercial models. The Honda GCVs are rather prone to having that belt jump teeth on either the crankshaft or camshaft sprocket, resulting in a retarded valve timing and poor, erratic performance. If your problem is erratic running, with substantial loss of power, rather than hunting, please say so and we can talk about how you check the camshaft timing (which is very easy to do on that engine, fortunately, since you may have to do it from time to time).
