I rode on a metrakit bike with a 12mm bing and it is decent (40 ish). That little carb makes the kit behave kind of wierd though. I noticed when going down hill at full throttle it would barley go faster than on flat land and would pop like i was engine breaking, it's like the hole is too small to allow any higher rpm's period. The damn thing sucked gas pretty good too, smaller carbs often use more gas contrary to popular belief because the smaller carb makes higher intake velocity using more gas. I honestly don't know how much this would affect similar sized bings but I know for a face that there is noticable difference mileage between a 19 and 21 phbg (5-15 miles per tank) the 21 getting better milage.