Rodney,
I'm using an OS No.8 Plug and a 10% nitro. During the construction I really made an effort to try o have the tank's outlet leveled with the carb spray bar, and I finally did, but even though the engine seems to lose some rpm when it is in a vertical climb. I installed a glow charge valve and think it might have something to do.
I have also noticed that using a Master Airscrew or an APC makes a great difference.
Before trying to put a muffler with all the modifications involved in it I thought a perry pump would be a cheaper and easier solution.