The F was the letter for the 80 models.
Leo
Scroll down through this XS650 Tech page and the float heights are there with the reasons.
http://www.xs650.com/forum/showthread.php?t=43
Lots of good info under the XS650 Tech tab at that top of the page
Your bike came with BS38. That does not mean that is what is on there now.Hi!,Howndo I know what carburetor model I have? My bike is a 1979.