I have an XT660r with roughly 19000km on it. The engine has developed a constant rattle at all speeds that you can hear with ear plugs in. It was very hard to pin the area down but initially it seemed to be RHS crank area, though you can hear it all over the engine. The engine does not use oil, spark plug is a normal colour and fuel usage is excellent.

After reading advice on this forum and watching utube videos of similiar machines with a rattle, I removed the RH sidecover for inspection and checked the crankshaft nut which was tight, though bumped the torque up to 120nm as advised. None of the other drive gears or clutch basket seemed loose when checked and I retorqued them with new lock washers as a precaution. The only thing I found was a bit of wear on the drive gear teeth on the crank that drives the clutch basket. The matching clutch basket gear was unmarked. A Yamaha mechanic looked at the gear when I got some other parts and said it did not need to be replaced and would not probably cause an engine rattle anyway. Put it back together and it still has that noise. It does not change when you put it in gear and load up the engine.

Other possibility mentioned was decompression mechanism. Removed cam cover and the mechanism counter weights seemed to have a lot of play when moved by hand. Is this normal, and could it cause a constant rattle ?. How to check decompression mechanism is very vague. Timing chain appears normal tension and valve clearances were spot on. What else is there to check?

Is there any knowledgeable fans in Brisbane who may be interested in having a listen?. I am a ex car mechanic and have seen inside quite a few engines in my time, but I'm running out of ideas with this one and they appear very simple motors. This is the second XT that I have owned. I also previously had an XT660Z Tenere which didn't sound anything like this one. Sorry about the long post. Hoping somebody can shed some light.

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Life is too short to wonder what if?
2007 XT660R, currently under reconstruction. Previous bikesTDM900, DR650x2, F800GS, Transalp and many others over the years.