Re: My brother's bicycle...
Welcome to the fun!
Sounds to me that the slide within the carb is sticking. Mine did that once before. If the cable is too long, you should see that by its ends, when its all screwed together.
Now if your cable is too short, all you have to do is carefully cut the cable housing off, from around the cable. The ones I got with my kit were like this and I cut about an inch off it. One way to tell if its too short is assemble the carb end first and when you go to assemble the throttle end, it won't make it and you'll be able to see just how much more cable you need. At least, that's how I did it.
Hope this helps.
Don't feed the bears....