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A mechanical clunk then it stops, but starts straight away

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I've got an 84 XL600r which has a very annoying random stop feature.  It will idle a while at any time but there will be a random mechanical clunk and it stops.  It will start again, but with only my right leg for starting it does get a bit tedious.

Have rebuilt carbs, replaced coil, plug and stator.  Have disconnected the 2 decompressors.  Have serviced the old girl including checking valve clearances.  Haven't swapped out the CDI as it cost a packet.

Apart from this it works fine and next weekend we're doing the Taffy Dakar but I know by the end of the weekend my leg will be suffering.

Any ideas? Or things to check before I pull out the engine and do a rebuild

Thanks

Pete

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Is there any chance that the clunk is related to the engine dying rather than the other way around?

Have you tried raising the idle a bit to keep it running?  That's what I've done to mine so I don't kill it while slow riding in the dirt up in the hills.

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Does it have an automatic decompressor or a preventer that keeps it from firing backwards?

An XR650L has these and does exactly what you say, if the idle is low and there is a misfire the RPMs drop low enough to pop open the exhaust valve, sometimes it'll even kick the engine backwards a little (reverse rotation).

Usually a small carb mix tweak or raising the idle a little will prevent it, assuming it's not a vacuum leak, weak spark, or ignition timing problem

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Looks raising the idle a smidge will be the first and easiest option.  It's got a laser end can which is a bit loud for me and I'm use to decrepit British singles which will idle at very low speeds - just like your average concrete mixer.

And starting without a decompressor is a pain but possible, possibly due to starting said British singles for many years and also cause I'm a bit over weight ;-)

Any other ideas for things that are easy to check?

CheersIMG_20170108_124020818.jpg.622f924350d993ba0e65421d522c35ed.jpg

Pete

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OK - an update.  Raised the idle to 2000rpm and went for a spin.  When idling there is still the mechanical clunk, a hesitation and in most cases there is enough momentum for it to carry on.  But it does always finally stop.

Listening to the engine the clunk sounds like it's coming from the head.  Any ideas?

I can see surviving the Taffy Dakar will be a challenge but will give it a go and take the engine out and the rocker cover off when I get back and see what can I can see.  Any ideas?

Pete

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