cholmondeley:

You don't say whether you have operated at full throttle before. I had a similar problem after engine rebuild and when the engine was beginning to be 'run in' and the max rpm increased by a few revs. It turned out to be a carburettor/venturi problem; as a suggestion I would get a competent engine shop to check your carb. There were problems when the single piece venturi was introduced some years ago and in my case it only manifested itself when the revs were high; some sort of turbulence set up in the venturi of the carburettor. On the ground full throttle doesn't necessarily equate to max rpm.