This is the 2nd time ive had this problem. 1st time: went trail riding up a mountain, half way up, motor quit, backfired and sputtered but wouldnt run. It sat in the shop for about a month, and I decided to bring it in. I tried one last time before I brought it in, and it fired up and ran awesome (after a month of sitting) 2nd time: this is about 4 months after the first time, got it up to top speed, and then I downshifted back to first gear and came to a stop, the motor quit and now does not run. I know the carb is clean so it is not plugged. Any ideas on what it is? Thanks