Could be a couple different things. First, check to make sure that all of the air ducts and carb connections are seated properly and tightened down. To do this, you may wanna spray starter fluid around on the outside of those connections. If your engine RPM's increase, you know you have a leak as it's sucking in the air and fluid somewhere.
Second thing to check is the carb slide itself. It may be gummed up with something, preventing it from sliding freely.
One other thing...check your plug. I"m wondering if perhaps you are running a bit lean? Perhaps it's not jetted correctly? Just throwing out all ideas since you stated it was like this since new.
Sounds like the slide return spring is weak. I'd pull it out and find a stronger one or stretch the factory one out a bit to give it more tension. Becareful doing this, as too much tension will cause it to have a bog right off idle. Its a trial and error thing....
I had a similar thing in a carb once and it was a stupid little crack in the boot between the carb and intake. Sealed that up and she worked perfectly. That's the thing though...could be any number of things, so you just have to start weeding out the possibilities one by one. The good thing though is that it's very likely something simple and cheap, so that's a good thing.
Tan is a beautiful plug color. Try that leak test and see what happens. I worked on a cycle this summer with the same response as well...shot carb cleaner around a carb boot and the engine revved higher suddenly. Yahtzee.
No not yet. I wrecked my bike last weekend. Luckily I was there to break most of its fall. My body isn't happy with me though. Anyway Im waiting for my parts to come in. Should be today and I'll get her back together this weekend and let you know how it went.