Lois,
It sounds like your clutch cable needs adjusting. The quickest way is to loosen the large round aluminum locking ring on the cable adjuster on the clutch lever perch (item #25 in photo below), then unscrew the adjuster (#24) to take up any slack in the cable and allow the lever to fully disengage the clutch.
If this doesn't help, perhaps the gearbox oil is incorrect - it might be a good idea to change it (your dealer can do this, using motorcycle-specific 10W-40 oil). It's also possible that there's something wrong with your clutch; if this is the case, it looks like almost all the parts are available, which is great.
Ray
Attachment:
1978 RM250 Clutch Lever.jpg [ 19.63 KiB | Viewed 3621 times ]