Xiodinc,
Remove your air cleaner and look down the throat of the carburetor. Twist the throttle and see if you can see the slide, or barrel moving up and down. It sounds to me like you might have some binding in your throttle cable or the slide is installed backwards. There are two slots in the slide; one short and one long. The long slot should be on the right as you look down from the top of the carb. The short slot, the idle speed slot, is, or should be on the left. Check for any binding in either the twist grip or the cable. You want long gentle curves in the cable. No tight bends or kinks. As for your wiring. I'm not sure what you mean by the ground wire. The engine is grounded internally at the magneto. The black wire is the ground and goes to the black wire of the CDI. I'm also unclear on what you mean by soldering the throttle cable to the carburetor. Help us out here with pictures if possible. Get back to us with what you find.
Tom