I just recently bought a 1990 XR600R (20,000km's on the clock) from a mate who reckons "it just stopped one day and I couldn't be bothered fixing it". The way he was describing what happened made it sound like the timing chain tensioner had let go. Bad news. However when I took it apart I found it in great condition with no apparant damage to the valves, bore, piston or anything else so I just replaced the tensioner and the standard things like rings, seals etc and fired it up.
It started fine around 6-7 times while testing things like lights and on the first run-in ride of about 5 mins it ran perfectly. All of a sudden when I pulled up it sounded like the timing chain was loose again. I have been told that the timing chain AND guides (which were in good condition) need to be replaced every 20,000km's or so but that sounds pretty rediculous as all the other hondas I have owned haven't needed it. Has anyone else had this problem? Anything specific that I should look for when taking it apart again?

the chain tensioner sits on a shaft, did you replace that as well? you need to when you change the tensioner.
if the noise is at low rpm's it might be the autodecompresioncam on the camshaft. to check remove the camshaft and push on the autodecompresioncam while turning the camshaft, if you hear a click it needs to be replaced if you don't you got to check somethingels.
i know a guy that instaled an extra manual tensioner, you can find some pics in this tread http://www.motor-forum.nl/forum/topi...lepost/4938729 unfortuantly it is in dutch. but it is a mod i plan to do on my bike