Briggs & Stratton 8HP Carb

Hi all, Jim from CT here. Trying to help a friend out with his older Coleman generator. It is an older Coleman, probably 1989, all green and that assembly at the bottom left of the carb bowl {with needle vale, spring and nut} is giving us some problems. Someone apparently cranked on that jam nut too tight and stripped something. It just keeps spinning and will no longer tighten, making the engine run poorly and eventually stall { vibration backs jam nut out further and further until it stalls}. My question is, what exactly may have, or is more likely stripped? The threads on the needle valve or would it just be the nut itself?  Trying to educate myself a bit before I go tear into it. Hopefully this isn't one of those hard to find parts! Below is a picture of what I'm referring to. It's the adjustment assy bottom left. Is this an air/fuel mix adjustment?
 
 

Actual need the engine model and type numbers but yes it possible the special bolt can be bad. I have seen aftermarket that were nit even good right out the package; best to order the OEM parts for flojet from Briggs supplier.