Hi!
Yes! How much nitro do you use??
Don't make any engine modifications before you have set everything right first. That is: Tank hight , tank size , type of tank, fuel, glow plug etc.
A 14x6 or 15x6 APC prop will work just fine!!
Here is my choice:
Tank size: 10-12oz if you use a large slow rewing 15x6 APC prop. 12-14oz if a 14x6 or 13x8 prop is used.
Best tank is a Tettra "Bubbleless"! Second..an ordinary R/C tank like Kavan ,Sullivan or Du-Bro... with Uni-flow set-up (Two clunks).
Tank position: Center of tank inline with carb fuel intake orifice when airplane sits horizontal...This is very important!!!
Glow fuel: 5-10% nitro.
Glow plug: Nova Rossi / Rossi 3, 4 or 5. Enya 3, 4 or OS 8...there are many others too.