The RM 250 is a motocross bike that has no odometer, therefore there is no way of keeping up with the mileage.
this is the correct procedure http://www.kick-fix.com/IGN_timing/8v-procedure.html
To do it right you need an exhaust gas analyzer. Install the carburetor and try to get it to a point that it functions somewhat normally. This is where experience comes in and you can figure out what...
it should