An 11mm carb should not be too big for it. The DLE 30 uses an 11mm carb. The Zenoah G-26 uses a 12.7 mm carb. I use 11mm on 20 to 25cc engines, and the 12.7mm on 30cc engines. 11mm should work fine on that OPS. But as I mentioned, getting ram air into the carb like you have with your particular velocity stack arrangement can cause all kinds of weird tuning problems....

AV8TOR