True purpose of the Mixture Screw?

Interesting conversation and I thank everybody for chiming in. I've been having some conversations with fellow bikers and I assumed the "Pilot Screw" just affected idle, that's it. However, some people have told me that the pilot screw does affect the secondary circuit when you are going, say, 70+ mph. So if I were to simplify things and ask the question again, does the pilot screw affect things when going 70+mph? Another way of asking this is: if I'm running rich from riding 70+mph would adjusting the pilot screw fix this issue or would it most likely be something else, ie fuel levels in the bowls, wrong size jets, ect?

 

I think of it in terms of throttle position. 70+mph with throttle wide open vs 70+mph with throttle say 1/4 open.
Attached pic may help.

 

So if by secondary circuit, you mean the "main" circuit, then no, at 70 mph the pilot circuit (in all its glory) has a minimal impact. Given the coefficient of drag on an XJ and realistic power output, it's safe to say you're not at 1/4 throttle at 70 mph unless you're going downhill. So you're on the main jet/later taper of the needle.

Now's the time where we ask what modifications have been made? Float height can impact fueling as well. So could jet size if you're made modifications to intake/exhaust from stock, or PO changed out the jets. Post more information about your setup and the wizards on this site can help you better. Altitude could have an impact too if not at sea level, but otherwise, if it's all stock, you shouldn't need to enrich the mixture, setting it up stock is your best bet.

 

That is a great image, if I recall it comes from a Mikuni technical/tuning manual, correct? If not where did you source it from?

 

The anser to your question above is here:- @chacal
https://www.xjbikes.com/forums/threads/cv-carburettors-how-do-they-really-work.129527/#post-653387

And to answer @ManBot13 , I'm not having any problems with my carbs, finding them both easy and a pleasure to rebuild, especially the Mikunis,

 

It's from Keihin CVK40 (the K is for Kawasaki). https://gadgetsfixitpage.com/article.php/the-keihin-cvk-40-carburetor

Same carb can be found on HDs CV40 (no "K").

 

No mods, 100% stock.