I finally managed to get my 81 F100 (inline 6 - Carter YFA Carb) running more smoothly but now that's it's back and working it reminded me of a problem it has. Simply, it idles higher in park and neutral. Is this a vacuum issue. Is this an IAC issue? Does this truck even use an IAC? Thanks in advance.

 

set your idle while in drive to around 550 RPM, your curb idle (while in P / N) should be around 800 or so. This is normal for an auto trans.