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I have a 1997 Yamaha TRX850.

When I open the throttle I'm hearing a thud knock sound near the left crankshaft cover. That sound does not always appears and I can't find any correlation between appearances.

Clutch and speed of opening the throttle didn't affect that sound.

In the meantime, the engine works perfectly well and has never been opened since 1997. I have changed the oil and antifreeze recently and seems like the motorcycle in a very good condition.

Is that OK for a TRX?

If not, what should I check to get rid of that sound?

Maybe it would be better to not open the engine if it works well?

UPD1:

The sound appears only once in the beginning of a process of opening the throttle. So the flow is as follows:

engine idle working -> beginning of opening the throttle -> BANG -> continue the process of opening the throttle (no thud knocks further)

The thud knock sound happens simultaneously with the roar of the engine only once.

The sound is very similar to the sound of smashing two heavy steel hammers together except it more thud. I discovered it only after the 4 months of owning. It's not so easy to hear because of the engine's roar. Only if trying to hear it specifically.

UPD2:

Here is the link to video https://www.dropbox.com/s/55omtvm9bpccy5e/20160419_175448.mp4?dl=0

That sound appears on

  • 0:10
  • 0:40
  • 0:56

Hope you will recognize it

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  • Can provide a more detailed description of the sound and when it occurs? Can you consistently create the sound under certain circumstances? If you don't use the clutch and open the throttle all the way in first gear, second gear, third gear, and so on can you re-create the sound? Welcome to the mechanics SE site! Cheers! – DucatiKiller Mar 30 '16 at 6:21
  • Thanks for a response! I've updated the post and tried to be more specific. Right now motorcycle is on winter storage. I will have possibility to recreate the sound and test motorcycle with gears shifting next week and will update the post right after that. – mef_ Mar 30 '16 at 7:46
  • Nice update. So this is very, very low in the RPM range, just off idle if I understand you correctly. If it goes back to idle and you begin to open the throttle can you ever get it to do it again? – DucatiKiller Mar 30 '16 at 7:55
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    @mef_ No worries, totally understand. If you can add anything else, that would be great. I"m in US and will answer tomorrow. In the meantime I think some others are interested in your question as well and will respond with something. You've written it very well and that attracts peoples attention. I've rebuilt that engine before and have had similar issues with a Suzuki TL1000S, a Honda RC51 and a Suzuki SV1000S. – DucatiKiller Mar 30 '16 at 8:43
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    I have similar issue with my sv650. My noise occurs around 1500 rpm and more so when cold. I can't pinpoint the sound, under the tank somewhere. My model has carbs, not injection. I'm wondering if there is some problem between idle jet and the main jet as I imagine 1500 rpm is about where the main jet kicks in? Given age of your bike I'm assuming yours is carb model too? – Paul Robson Mar 31 '16 at 14:35
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I'm not owning this motorcycle anymore. But anyway will post my experience of trying to solve this issue:

I went to different mechanics in my town and their opinions differ:

  • One said that it's certainly is not okay. And he wants to take a look at the "guts"
  • Others mostly said that it's okay and I'm throttle too fast
  • This is the sound of the timing chain in the valve train and it's need to be fixed

I didn't fix it and sold motorcycle as is.

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  • Thank you for providing closure. Feel free to click the "accepted" tick, located to the left. – Criggie Oct 20 '16 at 9:06

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