Newbie question about Kyosho GT-12 glow engine

[Eleck Rider]

Hi, I have a question about my old GT-12. I've recently started running nitro buggies (just for fun, I'm not a racer) and I'm having trouble with setting my engine up.
After a lot of tinkering I've reached a point where I'm not sure what to investigate next. The engine fired and ran fine but after shutting a wide throttle opening the engine would die. I've tried different fuels and glow plugs but it's now at a point where it only runs with the starter on the plug. I fitted an OS no.6, (a hot plug), and have tried Byron Race 20 and Model Technics Qwikfire 16 fuels. Mixture screw is 2.5 - 2.75 turns out. Compression is decent.
My understanding is that the plug is still too cool, but I'm surprised as the OS is labelled as hot. Do I need to go even hotter? Any thoughts or ideas would be greatly appreciated.
Cheers, Chris