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The TTR250 FAQ web pages -> TTR250 -> Repairs and maintenance -> Clunk diagnosis needed!
Post Info TOPIC: Clunk diagnosis needed!
TTRfan


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Clunk diagnosis needed!
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I have been contacted this morning by an owner who lives in Vietnam. See his message below:

"Do you have any idea what might be wrong with my bike. When I START the bike, it occasionally makes a very loud clunk or even double-clunk noise the second it actually starts running. The noise seems to come from the valve system or somewhere else from the top part of the engine. When it happens, it sounds pretty nasty and I'm sure that over time something will break. I checked the forum but it was all about sprag clutch which I already replaced. It doesn't happen every time, so locating the problem is pretty difficult. I'm planning a 6 week road trip to the north Vietnam next summer, so the bike has to be perfect. Otherwise I'll be stuffed for sure somewhere in the middle of nowhere. cry "

A new sprag has been fitted and, giving the owner the benefit of the doubt, I am assuming the boss on the starter gear that the sprag runs on was in good order.

So what could the clunking be - any ideas folks?

Brian



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TTRfan


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More information from the owner:

1. Changing the sprag clutch didn't affect that problem. Although my old clutch was busted. That job was done properly. I stood by the bike and watched the whole time. I personally checked every gear and part.

2. It makes the noise ONLY the second the engine actually starts. Otherwise it sounds perfect.

 

3. When shutting down it still makes a very slight clunk. But apparently it's normal. 

 

 



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peteBLUEttr


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Could be any number of things eh. Has he actually had the cam cover off to have a look? Clearances ever been done etc? Binding camshafts?

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Pete. South Somerset, England.
TTR_RIDER_VN


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Another mechanic actually checked valve clearings and checked the whole top part of the engine(or was supposed to)as he was putting it back together when he changed the base gasket about 10 months ago. According to him everything looked ok. He seems to know his stuff, but you can never be sure...

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TTRfan


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Welcome to the forum TTR_RIDER_VN biggrin

I hope that between us we can diagnose the clunk!

Brian



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peteBLUEttr


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Welcome Moufou. Would love to see some photos of riding in Vietnam so hopefully the problem will be cured!

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Pete. South Somerset, England.
mossproof


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If it happens as the engine starts, it suggests to me that as the engine begins to spin faster than the starter is turning it, there is some "slop" somewhere in the system that is being taken up - not necessarily the sprag (which should be preventing this overrun backlash from occuring anyway), but perhaps on the gears/bearings? Was the engine casing in good condition around the holes that the intermediate starter gear pins seat in? Even if the gears and pins looked in good condition, the soft alloy of the housing could be fubar allowing the pin and consequently the starter gears to clatter about. I assume the starter motor itself is bolted tight, and not slapping about in the hole where it enters the casing? o-ring intact on the end of the starter motor and teeth in good condition?
Is the bike a blue one? Does the same noise occur if you kickstart it, or bump start it?
Good luck,
Simon.

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TTR_RIDER_VN


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It's a white one. No Kickstart. When sprag clutch was replaced, everything looked really good. I think that the sound is coming from the top part of the engine. Maybe the valve case has to be re-opened. 



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TTR_RIDER_VN


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Locating the problem is difficult because it happens only occasionally. But when it does, it sounds pretty bad. Like piston was hitting valves or something. Which is not the case, because the engine sounds and runs perfect for most of the time.



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mossproof


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You're right, I think that's the next step, although I can't think what would make that noise up top at only this one specific time. Also check cam chain tensioner and that the camshaft sprockets are aligned, have all their teeth and have no snags that the chain might catch. Could you start it with the top cover off? (Make sure the frame/wiring above is clean so no dirt drops down. There should be enough fuel in the carb for it to fire. Beware stray spurts of oil!)
Again, good luck,
Simon.

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