Hi Aeronut. That does seem small for a 30cc engine. I would try at least an 11mm carb on it and you could possibly get away with even larger. The glow engines come with carbs that are a little smaller than optimum to assist fuel draw as they have no fuel pump. With a pump, one can go to a larger carb. Perhaps either you or whomever converted the engine just copied the size that was on the engine originally. You can go bigger, and you will probably see more power....

Good luck,
AV8TOR