KLF300 Lots of resistance on initial acceleration, feels as if brakes are jammed on but rolls in neutral ok.

I have just got a KLF300 and it starts well but when you push the accelerator there is lots of resistance and noise before the ATV moves, same in forward or reverse. It feels as if the handbrake is on and then it releases when there is enough power and you start moving, but when you stop the same occurs. Any helpful pointers on what it might be, makes it very difficult to reverse with a trailer as there is resistance resistance resistance and then suddenly it shoots backwards! If I put it in Neutral and push it the quad rolls absolutely fine.