I have this same engine(four of them) and they don't run very well at all with an 11x6 prop(APC) or anything other than an OS# 3, OS #8, or Enya glow plug. An 11x6 is too much prop for it. I also had a friend that always complained about how his ran but he always ran an 11x6 on it with a Fox glow plug. When he finally listened to me and changed to a 10x7 and an OS#3 he was suddenly very pleased with it.

Switch to an APC 10x6,7, or 8 and use a OS#3 glow plug and it will probably run 100% better. Then make sure you have the low end set correctly. Most engines arrive with the low needle on the rich side so you may need to lean it out a bit.