Le Fou,

I'd just go back to basics. I also have a Magnum .91 (in fact, I have two) and they both run very sweetly. I broke them in exactly per the manufacturer's directions which includes using a 14x6 prop. I think that a 12" prop might be too light a load... you want to put a load on the engine, but just not too heavy a load.

Do you have the manual? If not, I can send a .pdf copy to you,

Bob