Simon,
You dont say what type of fuel you are using. With full castor you will get quite a lot of carbon on the valves(backside) which will reduce the power output. How old is the engine (hours of use)? I have run many 4st engines for many hours and only needed to adjust the valves after about 1 hr of use and never needed to adjust after. As LOADING said Check the plug first even change it with a new one and check the engine. If you are running 10% nitro fuel you should not need the onboard glow.

Cheers.