Have 1999 arctic cat 400 idles fine but accelerate it sputters and backfires. already cleaned carb

Hi Raymond,
This sounds an awful lot like a safety switch issue. A lot of ATV's now have safety switches which need to be deactivated otherwise the safety switch sends a signal to the ECU telling it to impede or delay the timing. So on your model I believe you have to pull the clutch in before you attempt to start it. AND you have an emergency brake safety switch. The emergency break safety switch can be activated if the emergency brake lever has been removed or if the pivot point is heavily worn, or if the rigging is wrong. All of these things will cause the lever to contact the switch improperly. And so your ATV thinks the emergency break is on. Basically your ATV thinks you're crazy for trying to accelerate while the break is on so it impedes or delays the timing so you wont hurt the ATV or yourself. And that's why your getting a nice idle but backfiring and sputtering when you try to accelerate.
Hope this helps. Good Luck