[RuffRoad]

I just bought a 2007 GZ250 a few days ago. The top end noise was unbelieveably loud, however, I thought "It's just badly adjusted valve tappets". I measured the existing clearances and found: 0.005" intake, and 0.008" exhaust. So, I adjusted them (when cold) to : 0.002" intake, and 0.003" exhaust. Thought to myself, "This is really gonna solve the noise problem". Went for a gentle test drive, not exceeding 40 mph. The noise was still just as bad - even after the engine was brought up to full operating temperature.
Next day, I decided to pull the gas tank and get a closer look at the valve cover assembly. All the bolts were there, so I gently, gently, tightened them, just "a tad". Lo and behold, I discovered that one of the bolts refused to tighten - it just spun around uselessly. Obviously someone had sheared the bolt, and left the hex head and shank in place, making it appear as though nothing was amiss.
So, sigh, pull the valve cover assembly. Sure enough, the broken bolt was one of the bolts holding down the camshaft journal bearing. Only ONE bolt, on one side of the camshaft journal bearing was actually keeping the camshaft bolted in place.
MORAL OF THE STORY: If you are having intractable top end noise, and valve adjustment has no beneficial effect, trying gently cinching up the camshaft bearing bolts - you may find that one has been broken in half by an overzealous, wrench-bearing, ham-fisted amateur.