Nick, Spencer,

You are both using fuels that don't contain any Castor oil in them.

This engine is based upon the now defunct OS.40FP.

Most engine 'gurus' and even OS themselves, say that this type of engine must use fuel that contains some Castor oil. Some meaning at least 5%.

My own experience is a peeled sleeve, in an FP that was run on Cool-Power green.

BTW, without Castor oil in the fuel, there shouldn't be anything that would burn to form varnish on the head, should the engine be run too hot. Strange!