drac:
I was wondering if you have the copper crush washer that goes between the header pipe and the head. This is the copper washer that absorbs the compression (much like the washer or an automobile spark plug) when the "nut" is tightened. I have found that if you do not use a washer or if you use an old washer then the header nut will loosen. I install a new copper crush washer, tighten everything down, run the engine, retighten when hot (tighten the nut IMMEDIATELY after engine shut down). I go thru three of these cycles and I have had hundreds of runs on my YS 140 and 160 without problems.

Ed