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Engine knock

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Hi all, I have a 99 XR400.  It developed a engine knock a wee while ago.  Will only do it at relatively low revs with load.  

Will do it from cold.

From idle, give throttle a blip and it will give off three or four knocks or more like a clack.

From lowish revs while riding give it a resonable amount of throttle and same 3 or 4 knocks.

I could ride cautiously and not make it knock at all as long as not too heavy on throttle.

At high revs give it a fistful and no knock.

 

Started checking valve clearances, spot on.  

 

Check timing, spot on.  

 

Checked timing chain tensioner pushrod protrusion for length in case of stretched chain and is got shitloads of travel left.  Did find that if I hit pushrod in with heel of my hand hard it would jump back in so thought i had my problem found.  New tensioner (which i couldnt get to jump back in) fitted but problem still present.  

 

Removed camshaft and pressed off all the decompression carry on, refitted and problem still there.

 

Though i had covered my bases externally and decided to bike the bullet and rip head off, to surely find evidence of piston to head or valve contact.  Nope! 

 

Keep going and bore is sweet, have stripped to core and have only found in gudgeon pin some small amount of material pulled in the piston, very minor and pin has no noticable movement when in piston.  Expected to find a fair bit of play in the big end but seems to be tight as a drum

 

Anyone got any bright ideas?  Im a bit stumped and there is nothing worse than spending all that time, then the money for gaskets etc to put it back together to find noise still present.

 

Any input would be greatly appeciated.

 

Cheers

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Don't know if it will help but my '96 XR250 had a noise that I thought was the cam chain, took it apart, chain and guides looked good. When I removed the right side cover to inspect the tensioner I found a tooth broken on the kickstarter idler gear. It was a very light knock. I got lucky and found the broken tooth in the bottom of the crankcase and removed it with a magnet. Might be something to check.

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Have you tried fresh 91 octane gas?

Yeah, don't know what your options are in kiwi land, but here there are a few stations selling 100%, non-ethanol gas with 93 octane. Seems to be much better for carbs, no gunk, no water, no pinging or knocking for me.

 

cheers

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I don't know if my 97 XR has the same thing as yours, but it kind of pings, tings, ticks sort of, from the top end. Thought it needed a valve adjustment, but that didn't fix it. Bike's been running strong now for 3-4 years, pinging away. It's got to be the dad gum ethanol gas I've run through it. Even premium gas with ethanol in it sucks. Try to get your hands on some aviation fuel and see if it knocks then. I guarantee you it will not. 

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Use a dial gauge to check piston to bore clearance. Check for taper in bore.

Check shop manual for clearance specs.

Using a feeler gauge is not as accurate ( in case that's the way it was checked)

If it's questionable or at max specs,

Good time to replace piston and rings since it's apart anyway.

If all else checks out ok, that may be the problem?

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I had an XR600 years ago and while I was wringing its neck (reving the shit out of it) I broke a valve guide and it was a horrible knock was so bad I thought the piston was going to come out the gas tank. LOL

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I'd like to know the procedure you used to determine that you still had plenty of travel in the cam chain tensioner left, meaning that chain isn't worn out.

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