Hi John,
Hmmmm, I'll have to measure the setup on the one Hanno that I'm running. Overheating and blown plugs sure sound like your pipe length is too short. You're certainly not overpropping the engine. On my Hannos I found the needle setting got more critical as I increased the prop load (I've run a 13x11 on a Hanno - 13x10 is probably the max without going to 20% or more nitro). The Webra has about 33% more displacement than the Hanno. I've run the Bully 1.20 (basically a Webra) a lot and found it had MUCH different running characteristics than the smaller .60 sized Hanno. It prefers a bit more prop and lower rpm (15x 11 or 16x10 seem ideal) with a long pipe setting. Maybe your Webra wants more prop and a longer pipe length. I always run my engines as rich as possible. The Hanno hi-speed needle is much more sensitive than the one in the Webra carb, but I found I needed to lean the low-speed needle to get good mid-range transition and reliable high-speed needle settings especially with the Webra. I hope this helps - trying to fly pattern with a tempermental engine is just about impossible and dammed frustrating!!
Good luck and keep us posted!
-Will B.
PS I've been running piped engines since the late 1970s and I still don;t know everything yet :-)