It runs on mix - 50:1 so no separate oil feed. Almost all small gennies up to about 1.5kW are 2-stroke in UK

Air filter is brand new - and fault is present with or without air filter. Exhaust is fairly clean so mixture is probably about right, and there is no sign of four-stroking which is the classic indication of a slightly rich mixture on a 2-stroke.

Fan speed inside cooling shroud is the obvious idea Darowyn, why didn't I think of that? Mind you the lever seems to have a very powerful push for something on an air-flap.

I wonder if there is a flap damper or if there is supposed to be a friction damper or oil-filled damper (remember SU carburettors?). Much more dismantling required. The fault is a recent development, and when it started I too thought shit in carb or shit in airfilter. The additives in modern petrol are a real bugger for leaving deposits.

Governor is esssential on a generator where you leave it unattended and the electrical load can vary yet you need constant volts...