I got a used Keihin CV carb off ebay for my '69 XLCH 900. I messed with a few slow jets, shims under the needle and mixture screw settings. Got it to start easily (magneto), idle good, run good and get good MPG (mid 40s). But could not get rid of the occasional carb hesitation.
Read about the Velocity Needle on the XLFORUM so I gave it a try. It was the final key. Not only did the carb farts stop but I went down one pilot jet size to a 42. Starts, idles and runs great. Plugs are burning just right (Auotlite AT 42) and MPG is mid 40s around town.
I'm ordering one for my 2004 Roadster 1200.