I looked back through this thread to update myself on it.
Nevermind what I said about servicing the clutches. For some reason I thought it said "350", not 250.
The 350 is automatic. The 250 is not. Since you have a 250, there is no servicing of the clutches.
Since you have the same issue with 2 different carburetors, I suspect you have something else wrong.
First thing I would do now is check the valve clearances.
Intake clearance is: 0.002"-0.004"
Exhaust clearance: 0.004"-0.006"
Always adjust valves between middle to max clearance. Do not adjust them to the tight side.
If you have never done valve adjustments before, download a factory service manual for less than $20. The whole procedure is in there.
Btw, the Bruin 250 is basically an updated Yamaha Timberwolf 250 or BearTracker 250. Many engine maintenance procedures are the same for all models.
The Timberwolf was first. Then it became the BearTracker. After that, it became the Bruin. Then from the Bruin, it became Big Bear 250.