that can be frustrating...sounds like a couple carb adjustments.
the clutch engage pretty low on these at around 2100rpm. I am pretty confident that the reason it is taking off on you is that the idle is too high. It also appears that you have a dirty carb or your air screw needs an adjustment
On most of these little sleds the foam in the air box is removed to help increase air flow. this will help it breathe a bit better and move in the direction on helping your problems.
If this were my problem i would support the back of the sled so the track is suspended and not touching the ground. use a ratchet strap or something, just get the track off the ground. Next i would start the motor and let it warm up...these little sled can be a little cold blooded. Once it is warm, pay attention to how much you are turning as a reference point, and screw in the air screw until it lightly seats. DO NOT OVERTIGHTEN!!! The motor will stall, now i am guess the screw is more than a turn and a half...set your air screw to 1 1/1/4 turn open. Restart motor. You will have to adjust your engine idle again as it will now have change quite a bit...your track may be spinning at this point. Once you get the rpm back down, you will need to hit the throttle and continue to adjust the air screw to try and eliminate that bog. It can be a pain, life is a lot easier if you have a tiny tach on there and can see exactly what you are doing with the rpms...keep in mind you want your engine idle to be just below the point where you hear the clutch start to tick...
The clutch will start to tick at around 1900 to 2000rpms...(think these numbers are right, been a while since i have a played with a bone stocker) The clutch will actually engage about 200 rpms higher so it is a small adjustment we are talking about here.
The goal as i said is to have the engine idling and the clutch not ticking. If it the idle is too low it may just stall when you hit the throttle.
Keep in mind, the carb has two fuel circuits. A high speed and a low speed. At idle and low throttle the carb will be using the aircrew and pilot jet to get fuel to the motor, once you get the rpm up the main jet takes over and your problems go away...this is all tuning on the low speed circuit. If you are using anything higher than a 87 octane gas rating this will be a pain in the ass. Higher octane will NOT work in these sleds in stock form. If you need any more help, feel free to pm me and i would be more than happy to help you get through this problem....