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Yamaha Tech Originally the Yamaha XT600 Tech Forum, due to demand it now includes all Yamaha's technical / mechanical / repair / preparation questions.
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  #1  
Old 15 Dec 2012
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can the cam chain slack produce failures?

Hi, experts guys out there, I have a XT 600 and I rebuilt the upper engine, meaning cylinder, piston, rings, valves seals, seats etc. because of gray smoke. I noticed that the cam chain tensioner was all in (in the last mark) but I did not change it. After all that the bike had a failure at 1/4 throttle (2500 rev). It sputters and shakes as I accelerate more and more, I feel a lot power but a lot of vibration too.

Question: is it possible that the timing is a little bit off due to this slack?

By the way I check the carb, the plug, the gas, the filter and everything was right.
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  #2  
Old 15 Dec 2012
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Yes I would say that a slack camchain can slightly alter the timing, and if its really slack, what is it hitting against? A Yamaha 'expert' will be along soon to help you, in the meantime to the HU !
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  #3  
Old 15 Dec 2012
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As a matter of fact it is not so noisy, but becasue of this small sputtering someone pointed to the chain since everything else looks fine so far.
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  #4  
Old 15 Dec 2012
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Yes, the chain can jump teeth on the cam sprockets if slack and alter your cam timing; easy enough to check if the timing is out by removing the timing cover and lining up the relevant marks and checking the cam position.

It sounds like your timing may be out, but clogged jets in the carb could also cause these symptoms.

Regardless, I'd change your timing chain asap in any case before it snaps and grenades your newly rebuilt engine.
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  #5  
Old 15 Dec 2012
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Mine is a different bike, but my cam chain tensioner failed to automatically move in which caused the chain to become slack and the chain to jump a few teeth

The bike stopped due to the timing being seriously out, being a Vstrom and a Vtwin, the pistones were working against eachother

I have new chainbs and tensioners on order

replace that tensioner, it will cause you problems

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  #6  
Old 16 Dec 2012
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So if I check and the marks are out it means the chain jumped. And if the masks are lined up the timing is ok and the problem is another?
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  #7  
Old 16 Dec 2012
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Pity you didn't change the adjuster or more likely, the cam-chain "block" that places tension against the cam-chain when you did the top-end rebuild.

There could definitely be timing issues with a slack cam chain and "tooth jumping" is possible.

If it's an "interference" engine (meaning that the pistons and valves could touch if not time properly), yes, you could have catastrophic failure.

Is there currently cam-chain noise with the adjustment at it's present nearly full-in position ? If so, you need to address this SOON.
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Old 16 Dec 2012
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There is no noise coming from the cam chain and I dont think it has jumped a teeth either.
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Old 16 Dec 2012
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Quote:
Originally Posted by rich7 View Post
There is no noise coming from the cam chain and I dont think it has jumped a teeth either.
Tooth.


Have you checked your carb jets/spring/needle/diaphragm/intake & airbox rubbers?
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Old 16 Dec 2012
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Yes, tooth. Yes indeed, when I rebuilt the top-end I took the carburator apart to give it a clean-up.

Should I check it again?

Another related question: if the camchain has not jumped a tooth but it really has a slack (last mark of tensioner), can it produce the problem I described above?
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  #11  
Old 16 Dec 2012
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In one of the manual I managed to download there are these two images, Which one should I use to give the necesary valves clearance?
Attached Images
  
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Old 16 Dec 2012
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Mark 1 in first pic, the one which looks like "I" .NOT the "H"

Make sure you set it on TDC on the compressionstroke. You should see the intakevalves go down and back up, shortly after you'll have tdc on compressionstroke.
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Old 17 Dec 2012
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Then, what is H mark for?
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Old 17 Dec 2012
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It will probably tell you in the manual; but if I was a betting man I would postulate it is to do with ignition timing......

If your chain is not noisy, I doubt jumping a tooth is the cause of your problems; the chain needs to be changed all the same, though.

Did the problems just start suddenly one day after it was working OK, or did it never run right after the top end work?
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  #15  
Old 17 Dec 2012
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Thanks for answering this fast. No. this problem started just after doing the rebuilt. I just learned a couple of things: 1 I set the valves clearance using the H mark instead of the I mark, 2 I just rode my bike with the choke out and it runs definitely better. (see my other recent post)

I remembered that when I cleaned the carb we put the position of both needles on the last marks just because my spark plug used to be black and some people recommended me to work with the needles and the jets.
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