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New to me bike making weird noise?

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I bought a used 2007 kx250f about 2 weeks ago now. Everything has been going great with it except for a noise that started maybe 2-3 rides after I got it. I changed the oil and put in rotella 15-40 by the way. 

 

When ever the engine has a load on it, there is a slapping type sound but it almost doesn't sound like metal :/ it kind of sounds as if a piece of the plastics was flapping in the wind haha best way I can describe it. 

 

Biggest clue is that it ONLY happens under load. In neutral nothing...if you are just rolling and you shift up it progressively gets worse the more load on the engine as you get into higher gears. Once you rev past the point the exhaust has a deep tone it goes away. 

 

The manual says 95 octane fuel I'm running 91 not sure if that could be an issue? 

 

 

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Shot in the dark but it could possibly be your drive chain. I had a broken tooth on the front sprocket of my bike and the chain started to slap Against the swing arm

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That and if your lugging it in too high of gear. Like starting from a dead stop in 3rd gear. Then flooring it. It will make chattering sound. But that's not what I think this is. Good luck

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Shot in the dark but it could possibly be your drive chain. I had a broken tooth on the front sprocket of my bike and the chain started to slap Against the swing arm

Chain does have some excessive play I'll try to tighten it to spec and see what happens.

 

Any other suggestions appreciated. 

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I see you are in Canada.  I have never been so I am not sure which method they typically use to measure a fuels octane. 

 

  In the USA 95 octane would be higher than anything you can get at a common gas station. 

 

  Either way I highly doubt it is knocked caused by too low of an octane.  I don't know as I am not there obviously, but it is unlikely. 

 

  I have to assume (yes, I know) that the 91 octane fuel you bought is rated by the typical system of RON+MON divided by 2. 

 

  I would be interested to see where you found the 95 octane recommendation for your bike? 

 

  Chain slap was what i thought when I first read your post. Chain too loose, chain that is unevenly worn, bent rear sprocket or bent counter shaft. 

 

  Hope you get it figured out Boss. 

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I see you are in Canada.  I have never been so I am not sure which method they typically use to measure a fuels octane. 

 

  In the USA 95 octane would be higher than anything you can get at a common gas station. 

 

  Either way I highly doubt it is knocked caused by too low of an octane.  I don't know as I am not there obviously, but it is unlikely. 

 

  I have to assume (yes, I know) that the 91 octane fuel you bought is rated by the typical system of RON+MON divided by 2. 

 

  I would be interested to see where you found the 95 octane recommendation for your bike? 

 

  Chain slap was what i thought when I first read your post. Chain too loose, chain that is unevenly worn, bent rear sprocket or bent counter shaft. 

 

  Hope you get it figured out Boss. 

 

 

 

I took another look at the owners manual and it says RON+MON/2 minimum 90 RON minimum 95?? Not sure which it is. USA octane ratings are the same as Canada far as I know. 

 

I checked the chain tension and it is 2" slack which is within spec. I suppose it could be any number of the other chain related issues you mentioned. 

Edited by cric8192

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Chain slap is normally at low revs under load, if its doing it at higher revs under load then it could be pre ignition from wrong fuel being used.

 

Chain slap is easy to fix, remove the chain slider and put some automotive double sided tape under the slider about 2mm thick and it will stick to the swingarm and cushion the sound , it will also stop excessive wear on the swingarm.

Edited by Shawn250

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The RON + MON divided by 2 is most likely the number listed on the fuel pump you use. If you look at the actual sticker it will either show you RN+MON over 2 or it will just say RON. 

 

  If you are buying 90 or 91 octane from a normal gas station I have to assume they use the same method  as we do for octane rating.

 

  Typically the factory service manual erros on the side of caution, and will call for higher octane in many instances than is actually needed.  In a stock bike 91 is more than plenty of anti knock for most people and most situations. 

 

  Best thing would be to try and get a video showing the noise, then we could potentially be more help Boss. 

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I think I figured this one out! There is a roller type chain guide that wasn't spinning so the sound of the chain just dragging over the teeth of this roller was making the noise. Back to sounding like a healthy 4 stroke again

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Shot in the dark but it could possibly be your drive chain. I had a broken tooth on the front sprocket of my bike and the chain started to slap Against the swing arm

My chain was a little loose. Did exactly what op is describing

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