Mr fixit,

I was looking at my. 12 carbs and none of them are small enough to fit your .21 engine. The smallest .12 carb I have is 10mm neck (outside, thats my 10E carb for my OS .12cvr ) and they go up to I think 13mm for my NR .12 . These are carbs for .12's and you are looking for a .21 carb which i would bet it bigger for sure. One of the smallest .21 carbs i can think of is the K&B Black and that carb has an 11mm neck.

Hope this Helps,

Paul