I recently got a '91 xr600r. The previous owner had installed a new piston and I'm not sure what else. The engine starts and runs o.k., and seems to have good power. The compression, as measured by the feel of the kick starter, seems lower than that of a '93 I used have. There is an intermittent "clicking" sound coming from high up in the engine. Sometimes I hear it 10 times a minute, and then sometimes I won't hear it for 5 or more minutes. My guesses as to the source of the noise are the decompressor or cam chain tensioner. I know trying to diagnose a sound that you have never heard is difficult, but does anyone have any ideas?