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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 6 Jan 2008
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XT600 rattling valve

Hi all,
been reading stuff here for a while, and plucked up the courage to register (since I don't travel the world yet)

anyway,
I have a 1986 XT600 (early kickstart only model) with around 40k on the clock.
Was very noisy from the top end so I decided to do the valves, although the bike was still running a treat and pulling like a good un.
Anyway, one of the exhaust valves is as loose as anything, can move it up and down a good 1/2 cm.
I did all the gaps, as best I could with that one, and put it all back together (my lets see what will happen philosophy)
Bike fired up from stone cold better than it ever has, and settled into a lovely tickover in no time. Top end is quieter, just ding ding ding ding from the one valve. Revved it to 4k beautifully, and seems to be running spot on, apart from the noise.

So, to get round to my question, why is it so loose? And what damage would it cause running it like that? Also, if anybody knows how I fix it, that would be a bonus.

Cheers,
Stew
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  #2  
Old 7 Jan 2008
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StewH
I've only been as far as France/Italy on a bike, and usually camp in the Scottish Highlands (good trout fishing). Doesnt stop me posting - don't let it stop you.

Just to be clear - do you mean the rocker arm was moving freely up and down, or the valve itself. If the valve is moving easily by hand it sounds like the spring is not working, If it was a huge valve clearance on the rocker, then I suppose it's possible that the rocker was thrashing up and down and barely operating the valve ie you've been working on half or less of valve opening. It would certainly bash down noisily on the end of the valve stem. If so, it would be worth a good look at the rocker arm to see if it's been deformed.
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Old 7 Jan 2008
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thanks for getting back to me on this.

to clarify,
when you have the motor at tdc, all of the rockers should be slightly moveable according to the clymer manual. The right hand exhaust one tho, is much more than slightly moveable, I can pull it up about a centimetre and it would stay up (I think).If you stick your fingers in, and grab the locknut, you can move the rocker up/down with ease. But only on that one rocker. The rest just have a tiny bit of freeplay, as described in t'manual.

now I am not really very mechanically minded, so I only go by what is in the manual, and once I've exhausted that (pardon the pun) then it's just down to my imagination and lack of confidence from there on in.

I was gonna take it for a wee run this morning to see what it felt like out riding, but it's 60mph wind and horizontal rain at the moment.

What is surprising me is that the bike feels miles better to start etc. All the clearances were too tight, so I have set them to spot on (as best I could with the wierdy one)

Sorry it's all a bit vague
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Old 7 Jan 2008
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I wonder, could this be anything to do with the decompressor? Does anybody know if it works on both exhaust valves or only the right hand one?
I am guessing that it holds the exhaust valves open a wee bit to make kicking the bike over a bit easier, so if it was incorrectly adjusted, might it hold the valve open more than it is supposed to?
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Old 7 Jan 2008
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What boxer said. I'd be suspicious enough to check for deformity of the rocker arm. 0.5 cm free play even in the loosest setting is a pretty big deal. And yes, the decomp only actuates the RHS exhaust valve. It may be worth checking that you didn't set the clearances with it engaged.
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  #6  
Old 7 Jan 2008
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I'll tell you what, I could almost guarantee that I did it with the decompressor engaged. Why do I know that? because I have no idea how you would disable the decompressor.

So, can anybody explain how to do the valve clearances without the decompressor interfering?
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  #7  
Old 7 Jan 2008
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It's not something you should have to do. Check there is enough free play in the cable. I dont know the manual specs, I have about 3mm free play on the lever on mine which seems to be enough. Also make sure the lever springs forward after release and is not fouled and become "sticky". With the exhaust tappit cover off, push the lever back and see it depress the valve. That way you know for sure it's either on or off. Also, and I have done this a couple of times in error, make sure when you do do the valve clearances that you are on the correct stroke. It's a 4 cycle engine remember. The crank revolves 720 degrees before a cycle repeats itself. In other words you can set the timing at TDC and set the clearances correctly, revolve the crank another revolution to TDC and suddenly find all your clearance settings are too loose. It's a pitfall that I personally am always foul of. Then you should hear the language :-)
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  #8  
Old 7 Jan 2008
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stripped it all down, double checked that I was on tdc (t mark aligned with crankcase mark and all rockers have movement) , set gaps to .10 mm inlet and .15mm exhaust.
Bike is running a treat, just still dinging away like a good un.
I checked over the decompression cable, didn't seem to be any free play in it at all, so I've put a tiny bit of movement in it.

Ah well, I'll find out tomorrow when I go for a good long ride on it.

I reckon that other than this setback, I could start to finish strip it and adjust the clearances and put it back together in under an hour. Longest part is removing the seat, I'm gonna look for some nice knob ended screws to put in there instead of the torturous fiddly wee 8mm things that hold it all together at present.

Thanks for your help folks, I'll post the ride test results tomorrow, fingers crossed gales will be gone by then and I can get stuck in.
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  #9  
Old 8 Jan 2008
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well, did a quick 15 miles commute, bike appears to be running absolutely fine, but the noise from the top of the engine is unbearable.
Looks like I am taking my xt out of duty until I can afford to strip it down and see what's what, or get a mechanic to sort it.

Thanks all, and orra best
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