Check the cables by taking the throttle tube off. Pull the cable and let go. If it snaps back into place it is not a cable issue.
Remove the throttle tube. Sand your bar down with a fine sand paper or steel wool until perfectly smooth. Clean the inside of your throttle tube with brake cleaner... do it several times and wipe it out with a paper towel. Once the tube is clean and dry spray your handlebar lightly with WD-40, Don't use grease or a heavy motor oil, use WD-40. Put the throttle tube on and see if it spins freely. If it does, connect the cables and as Evenslower said, make sure there is enough space so the end of the tube is not rubbing on the end of the bar.