Use a real carpet knife, they are double sided and twice as sharp as a ulitiy knife with isn't sharp enough for carpet.

You can buy one for pennys from any hardware store.
I have been using one since 1978 when I took "floor mechanics" when I was in hi school. I use it for tons of stuff because utility knifes are dull compared to a carpet blade.

And carpet knife is shaped different and is much better.

Did I mention I kicked carpet for a few years to make a living?

Oh yea...real race cars don't have carpet.