OK your running late mc21 carbs same as my 93sp those bits that the blue marked hoses are on don`t have any hoses on them on mc21 so i guess air should not go through . The plumding on a mc18 and mc21 are different . See here http://tech.nsr-world.com/nsr/mc21/mc21-air-correction/ and here http://tech.nsr-world.com/nsr/mc18/mc18-air-correction/ . How you should plumb them up is a good question someone on here like Andy or Matt probably would know the best way to do it . There are people running mc21 carbs in mc18