Why does my 98 arctic cat powder special EFI 600 backfire after i start it up? I have no power at all.?

That could be many things but I first would try new spark plugs and be sure to use the right plugs for the engine. It also could be a timing problem if anyone has pulled the stator plate. Check gas lines for air bubbles. Make sure your muffler doesnt have a mouse nest in it.