It's a Thunder Tiger engine, MinnFlyer, so you know it's a good engine!

That little sliver of open carb at idle is normal. If it were closed completely, then the engine couldn't idle after being started on the ground. For a standard radio without a throttle kill setting, this slight opening should be present at minimum throttle but closable by turning the throttle trim all the way down. This will allow you to turn off the engine from the radio.