I had this issue once, see if this helps. I remember it being....
your clutch adjustment. Loosen it entirely and follow the manual instructions for how to get it back to the correct length. You have to loosen the adjuster at the bar, then uncork the cover on the left side. Wrench the nut to the left (takes effort if I recall)
Once it's loose you will slowly tighten the Phillips head screw inside until it's just starting to get firm, then loosen it a 1/4 turn. Then you need to keep the Philips in place and move a socket over it while you tighten it. I did the same by estimating how far it turned and compensated by loosening the Philips the same amount. so if the turn of the nut to the right is 1/4, I added a 1/4 loosening to the Phillips. When I tightened the nut, the Phillips goes with it. You'll figure it out.
After that readjust the handle bar tension nut on your clutch cable.
When the clutch is out of adjustment it will allow the engine to jam and the bike will stall every time. This drove me mad the first time.