The 68S will work wonders. One thing that I think you need to do is check the float level. From what I know it seems like you are using really big main jets. So big that if I tried them in my XR600 it would not run well at all. I would have no top end at all if I ran a 175. Perhaps the fuel level is low in you carb. The really big jets would then compensate for the low fuel level. Just guessing but I would expect a 158, 160, or 162 would be best.