12 mm. A struggle, given the cramped space you have to work on it with carb mounted.
Some have used bent-tip long-nosed pliers successfully for turning the plastic nut.
I've thought of using a crow's-foot wrench on the annoying little fastener, but never any success with that scheme. I've found, finger-aligning (while pulling the epidermis off my finger tips) a painful but sometimes necessary procedure for installing the nut correctly.
Check for cross-threading, cracked plastic.
Steadman replacement metal nuts are 1/2" hex, IIRC.
“You better put down that gun. You got two ways to go, put it down or use it. Even if you tie me, you’re gonna be dead.” "John Russell" (Paul Newman), Hombre