My daily ride is an XT500, 1977 model. Talk about your original Thumper! Anyway, I have a very loud ticking/tapping sound coming from the head while she's running. It's RPM specific, i.e., when the RPM's increase so does the frequency of the tapping. I have adjusted the valves, they were fine. What I did determine was that if I pull the de-compressor lever in slightly, the sound goes away. But anymore than a slight pull and the bike immediately loses power and wants to die. Thoughts and/or suggestions? We get this resolved and I'll ask about a cold engine miss/hesitation...Thanks!