In reverse, I can go slow, about 5mph, but soon as I go over that speed, it starts to sputter and shudder and thump..

Anyone know what it could be?

 

Your reverse rev limit, it keeps you from going too fast and wiping out if you turn the bars too quick. Some people can't handle going backwards fast, so most quad makers lawyers told them to limit the speed for safety reasons. I believe there is an easy way to bypass the limit, but I don't know myself, I have seen it on the forums, try searching for it, or wait and one of our many "tinkerers" will tell you how they did it. Being stuck in mud and not getting decent power to the wheels to back out would really tick me off!!

 

The weird thing was, it was working a few days ago, but it was all of a sudden it started.

Maybe i hit sumthing by mistake and it activated it.