Hi Neil,
I'm not expert whasoever, but I have a friend that has that same engine and he is having a lot of trouble tunning it.
I read on different forums to try to help him and found out that apparently is a very well known problem the tunning of this engine.
There is a modification for the carb if you look around you might find it I tried but I don't seemto be able to open the right page
You can read here http://www.runryder.com/helicopter/t...ight=os+46+mod
I hope that helps,
On another subject, typing all in Caps is considered shouting (just so you know)
Fernando