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It sounds like you have found your solution. Just my $0.02, with new air filters, and a carb rebuild kit with jets that are the same, or close to the same size, and engine losing power above 6k, when running perfectly before, that will always be a main jet issue (assuming you did the work right). RPM isn’t how you actually diagnose the carbs, but with these CV carbs, there is a strong correlation. In general, you can always tell Rich vs Lean by how the bike sounds just before it starts losing power. If it sounds “heavy” or slow to pick up rpm, it is running rich. If it sounds “light” like it revs easily, but there is no power in it, it’s lean. This is actually why we use these terms. Think of it as; Historically, Rich people could afford to eat, and were therefore usually slightly heavier (think women in portraits from the 1500s). While poorer people were usually a little Lean, and if too Lean (emaciated) would tire easily.

So you know your main Jet was too lean. I personally like to have a variety of jets on hand for various seasonal weather conditions (here in MI, we can have 3 seasons in one day). If you know your bike is running lean, you can always bore out your main jet. Especially if you have the old ones on hand and know they’re good. Obviously the size stamped on the Jet will no longer be accurate, but it is a good way to solve the problem without buying more parts (if lean). Keihin jets are sized in hundredths of a millimeter. So a 100 jet has a bore of 1 millimeter. Micro drill bits are sized by wire gauge. So run the conversion and you’ll get your new Jet size (maybe add a little bit after the conversion for inaccuracies of drilling by hand [literally] haha).
Since you are new here, this is what you do next.
 
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