Your performance numbers are a little low. The carb size is close to being ok, so I wouldn't worry about that for the moment. But you say it is 10.5mm. Where are you measuring it? A carb size is determined by it's smallest diameter in the venturi. What your engine needs is an 11.1 mm venturi carb. The Walbro carbs also have a number cast into them near the choke. The number you want is "28".

The next thing you need to do is add either one large or two somewhat smaller exit tubes to the muffler. If you use two they should be around 11mm i.d. One large tube would also work, and it should be around 19mm i.d.

These changes will help a lot!

By the way, your plane looks very nice. Good job!

Good luck,
AV8TOR