Thanks for that info . I was thinking it was for your float valve .
I thought that size was small , as the early 70's 360's use a 3.3 , but checking my 400 B parts book ,
that says 2.5 is the correct size . My workshop manual specs has the needle set on the 4th clip .
The w/s manual lists the needle as 6F9 . I don't think you've actually said what you needle code was .
Just enquiring as many of these bikes have been fitted with " carb kits " ,
and the accuracy of the components can be questionable . Needles especially so .
If your needle is something different , that may be the source of your ill's .

I believe this may be the missing washer MarkT was referring to. There should only be one washer here, the fiber one. You will want to set the float level once the brass washer is installed in the correct location.

Yes needle is 6F9, the carb seems to be original

You are right in that there should be just a small fiber washer under the float valve, but you are wrong in suggesting there should be a brass washer there, too. MarkT's posting was absolutely correct in where he circled in red there should be a heavy (by comparison to the small fiber variety) brass washer under the main jet.

I haven't seen the original poster Paulfb comment on the missing brass washer yet... Not sure if post was missed, not understood, or just ignored? Or maybe it's been fixed and not mentioned?

Without the washer, the needle jet can slide up which will lean out the mixture. I had one where the jet would slide up when running and then drop back down when shut off so it looked fine. That missing washer could be the whole problem.

Also, Snglsmkr noticing the washer incorrectly under the float needle seat could also contribute to lean running at high RPM. With that washer spacing the seat up, at full float drop the float needle will not be open as far as it would be without the spacer washer. This could cause the float chamber to run low on fuel at high RPM/throttle. Also the main jet is higher than it should be which compounds all of this even more.

Do you think the previous owner switch the washer in the wrong position?
So I have to move that washer from the air jet to the main jet?
Thanks!

If you mean move from float needle jet/seat to main jet, YES. Hard to see for sure if that's the missing main jet washer but it sure looks like it. Right now if you pushed on the main jet, the whole tube would move up into the carb throat. Bad! Someone else posted what main jet should look like with washer under it.

You will also need to adjust your float level by bending the tang after moving the washer. I added a red arrow to Snglsmkr's pic to show where washer belongs.

Sorry for the abbreviated observation that may have been confusing to some. The post was to highlight that there should only be one washer in the indicated location and not two. The fact that the extra washer was brass, I felt it could be the one MarkT had highlighted was missing from the needle jet/main jet connection.

Mods can delete my post, then. Looks like Paulfb knows what has to happen, now.