Reply To: Sprocket nut
It means you will need a new nut and a new washer (look at our shop bove ^)
And look at customer pictures, the nut needs to be 60 ft lbs torque tight ==> then fold the lock washer 2x to stop the nut getting loose ,
The nut fell out because nothing was holding it anymore (the washer failed its job or it was not tight or folded properly)
Also put fresh oil again (see tutorial below) and check the engine runs again before riding it. Check for leaks again.
If it is leaking again, when it is idling, = take a picture and show us.
[Tutorial] How to Change Oil & Filter on Hyosung GV + GT125/250 +R