RE: Unreliable engine
Gareth,
What type of OS Max .40 engine is it? Tell us that, so we can better analyze what is wrong...
There are certain 'musts' that apply to all engines...
The fuel-tank must fully isolated from the air-frame with 1/4-1/2" latex foam and be installed as close as possible to the engine.
This prevents the clunk from being deflected into the air above the fuel and ingesting air, by engine vibrations... as well as making the fuel-draw effort minimal, with the plane's nose up.
The tank center-line must be at the same level as the middle of the carburettor's barrel - this make the engine behave the same in upright and inverted attitudes.
If the engine is new, how did you do the break-in? If it is a non-ring engine, [link=http://www.rcuniverse.com/forum/Tapered%2DBore_Engine_Break%2Din_%2D_Upgraded/m_1850473/tm.htm]this[/link] is the best way; better than what the manual says.
What propeller are you using?
We'll carry on as you add more info...