Hi there, we are new to forums and would really appreciate any help people can give. We recently purchased a 250 trail boss 2x4 and after a few times riding around the paddocks the engine started excessive reving and forward momentum in high gear only. When you back off the throttle it will start to move forward again but it sounds nasty. Do you have any ideas??

 

Bad belt or secondary(rear)clutch or both

 

Thanks for that.
Is there any chance it could be the gearbox?

 

possible, but probably not
how does the belt look?
thats what I would try first IMO

 

The belt looks worn on one edge, checked belt tension and found it to be 1 1/8 inches
I have taken the driven clutch off to adjust belt tension, and found only one shim in there. Workshop manual says to leave at least one shim so I did only to find belt now has 1 1/2 inch deflection which looks too much.

 

it's possible for the belt to stretch out
I would start by replacing belt even if it is only worn on one side, and not stretched, I would replace it IMO

 

replace the belt

 

Thank you all for your help will source a new belt and try that.

 

Thank you so much to those who helped with fixing the problem with our bike. My partner found the gear selecter was not properly in place when he removed the gearbox. It is now fixed and running like a dream. Thank you once again.

 

you should have checked the secondary buttons when you had it off as there most likely starting to wear out as well.