Dunno Sachs that we’ll, but if it’s missing the spring pictured I think that keeps the starter mechanism disengaged until you pull the starter clutch lever to start it. So if that’s hanging up w/o the spring, maybe your clutch is engaged or partially engaged at idle? Meaning on the stand it can spin the wheel and idle ok but when you’re on it it can’t spin the wheel so the engine dies…
Just a guess without knowing more about the issue