Kawasaki Bruteforce 750 problem while running for a few minutes!?

On my Kawasaki Bruteforce 750, I just rebuilt the dual carb, now it will be running for a few minutes and then it will make a pop sound and the engine will engage the second cylinder and then the engine starts to rev higher without any touch of the throttle. I noticed there was a crack in the muffler by the cylinder head, I am going to replace that. Now I am checking one of the cylinder heads closest to the back of the atv to see if any of the gaskets are broken or cracked. I can't seem to find anymore problems with the cylinder head. What else could it be? Could it have only been the muffler that was causing the problem? Leave your email please so if I have a question I can message you. Thank you