Hi!
When you say that the engine starts to blubber (go rich ) when you open the high speed needle just 2 clicks I get a bit worried about your engines condition (piston to cylinder fit), because using as much as 15% nitro usually mean that you have a very broad needle range (means that the high speed needle could be turned nearly a complete turn without the engine losing much power.
Over here in Europe many of us use only 5% nitro in our engines regardles of make and have no difficulties with our engine settings. Using as much as 15% nitro is overkill for most engines and will sometimes give engine problems due to too high combustion temperature. Your OS is not one of them though and will run fine on 15% nitro But will run just as fine on 5-10% nitro.
It would help us if you could show us some pictures of your engine...and engine set up, and a picture of the cylinder and piston.
Regards!
Jan K
Sweden