Engine reving

I'm sure that this has been discussed in the past but I can't find it now. I just finished up a new bike and took it for a test ride. the motor runs great but at stops with the clutch pulled in the engine is reving uncontrolably. My first thought was that the idle screw was not set properly, this isn't the case. I tried ajusting it with the engine running and I could get it to stall but as soon as I re-started it, she would rev again. I even turned the screw all the way in and the bike started and it reved. I know its in the carb but not sure where. Any help would be greatly appreciated, I'd like to get this one on the sale block ASAP. BTW this is a 55cc GT3 (basicly a 48cc Spitfire with the hop-up jug) I haven't noticed anyone else here mention that they have one of these yet...Kelly

Thanks guys, my throttle seems to work fine as long as the motor is under load. After writing this post, I went out and took a walk and it came to me, I think the float may be sticking. TomarrowI will tear into the carb and weather its the cable or float, it will be fixed. Thanks again for the replys...Kelly