I'd already checked for that, but I thought the check valve in the gas cap shoulda been passing more air, so I experimented some more and ran up and down the street with the cap completely off and it didn't change anything.
I had noticed the air filter was deteriorating and had already ordered one when I saw your post. I was hoping that since I'd noticed something similar to what you described we would have a simple fix, but no joy. It actually may be running just a bit better, but the basic problem is still there.
I'm pretty befuddled. If anything, the float level is a bit high and I'm getting some popping in the exhaust as if it was rich, but there's no black smoke and the plug has a really nice tan color. The bike starts very easily, accelerates nicely and putters around just fine, but If I maintain anything from at 5 - 6000 RPM up, it will act kinda like it ran out of gas pretty quickly. Sometimes I can let off and it will stay running and other times it will die. It takes a minute before I can start it again if it dies. There's a particular sound this bike makes when it runs out of gas and I'm not hearing it. Instead it just kinda rolls back the rpm. On the rack, I can run it at those RPM's and the engine speed will vary a few hundred RPM, but it will never die. I do hear some popping in the exhaust. I checked the fuel level in the carb while it was at about 5500 RPM and it looks good and steady.
I'd pull the carb back out, but other than checking the float level, I don't know what I'd do to it.
Do you guys check the float level with it upside down, or do you hold it at an angle with the float just on the needle as some folks describe? I'm up for anyother ideas you might have too.