I have a 1999 900 STX with 3 Keihin CV carbs. It idles fine but when I try to accelerate it won't, it tops out at 3500 rpm. I've discovered that the diaphams in all three carbs aren't opening. If I take a screwdriver and manually open these "plungers" the ski takes off but as soon as I return to idle speed the problem repeats itself. I would automatically change these diaphams but they cost $125 a piece. What else could it be?

 

I see it all the time. Replace the diaphrams and guides it will fix the problen for shure! just watch when you take the covers off there is a long spring that will fly out if tour not careful.