you can unwind it but that's not too good of an idea. If you have a really really good pair of wire cutters you can nibble away at it, but you will probably damage your brand new cable.
If you cut the cable housing how are you going to take up any slack.
Before you loose any more length, have you tightened all the adjusters down? The "noodle" on the top of the carb should be threaded all the way in. Also make sure that the cable end is seated properly in the throttle slide. It may be stuck on the bottom, instead of near the top where it should be.
Most cables have some sort of adjuster that you can use to take up slack, just like bike brakes.
I'd also make sure that you have the cable set properly in the groove in the throttle.