A couple weeks ago, my Dad and I adjusted the valves on my '93 XR600R and 3 of the 4 were perfect, however, for some reason, the left exhaust (non-decompressor) was WAY out of adjustment. I mean, you could have fit two or three quarters in between the tappet and the valve head. We adjusted it to the factory .005in and it ran great. Then, this last weekend at the 49er enduro, I noticed ALOT of noise from the top end. I suspected the same exhaust valve as the culprit, and sure enough, today I went to adjust them, and the same valve had almost the same gap between the tappet and valve head! ***??? The lock-nut was still tight, but it was as if we hadn't ever adjusted it! What the heck happened? What does this mean and how can I fix this???