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Help diagnosing metallic pinging noise.. Pinging, Piston Slap, Or..?


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Hello TT universe… 

I've come across an interesting noise, and I can't quite pinpoint exactly what is causing it. It's a metallic pinging noise, it sounds like it's coming from either the front or top of my cylinder. It happens quite loudly at low rpms, especially when cold, however it does seem to continue even once the bike is thoroughly warm and ridden for a while. I uploaded a clip of the best audio i could get to youtube at this link : 

 

You can really hear it around the 4 sec. mark.

The bike is a 2004 husqvarna cr125, 75 hours on the current vertex piston, 19 on the ring set. I run 91 pump gas mixed at 28:1 with Torco GP-7. 

So as usual, I decided to hit my marks to see if I could figure out what was going on. Pulled the top end apart and inspected the piston and spark plug for signs of a lean condition. 

Piston head shows clear carbon build up right around intake ports, and less build up around exhaust ports, which kind of makes sense. Picture also attached. Spark plug is chocolatey brown. Does not appear to be excessively lean, so I feel like it may not be detonation?  Performance wise, the bike is not at all sluggish or boggy. Very responsive and smooth under heavy acceleration and when I need to access mid-rpm torque up through over-rev. 

This bike also eats up exhaust o-rings with the FMF pipe. I've adjusted the metal tabs for the rubber damper mounts on the expansion chamber, and since then its not as bad. I checked over the exhaust system before the tear down and all bolts were tight, exhaust was securely in the cylinder. I was thinking maybe it could be the exhaust rattling around.

Maybe the piston is worn past tolerances and resulting in extra noise? 

Anyone have and ideas? 

 

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Post a close up pic of the spark plug, 

detonation will sometimes produce tiny metallic particles embedded in the insulator but in this case it doesn't seem too severe.

 

Check your ignition timing to make sure it's not too advanced.

It might not be hard MX ridden but IMO 75 hours is quite high for a small bore piston, considering it's a 2004, how many hours in total?

Perhaps the cylinder is out of spec and requires replating, allowing the piston to 'rattle' or rock in the bore. (post pics of the skirt)

 

I'm also in the camp of 'more oil is better' (I premix my YZ125 at 30:1) but keep in mind more oil may also lower the octane rating of the mix.

With the questionable quality of modern gasoline, myself always blend in a bit of race fuel as a 'safety margin' in the event of a bad batch of pump gas.

Not much is required, say adding 20% of 110 octane to 91 premium bumps the octane in the 94.8 range.

 

 

Edited by mlatour
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Thanks for the reply,

Piston and spark plug photos attached. Although, spark plug photo may require some zooming to see details. 

This current crank was rebuilt with OEM components 104 hours ago. At the same service interval, cylinder was re-plated by millennium tech. Spent 29 hrs on a Wiseco piston, discovered excessive chaffing even with proper warm up, so I swapped to B size piston from vertex, which is same size as the OEM piston the bike came with. At 24.1, and 40 hours, piston ring was replaced. Hour measurements are exact, as I do keep detailed service logs of maintenance on the machine. 

Service manual for this model recommends 28:1, which is a bit high, but even with my high intensity riding style my crank has never posed any issues. Over 100 hours on this one with no play on the big end of the con-rod. 

Also, am I crazy, or did you hear what I'm talking about in the video??

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Edited by asapmeech
adding information.
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No didn't really hear anything out of the ordinary in the video, if detonation it's not severe.

Hard to tell with the plug picture, look deep into the insulator with a magnifying glass under a light.

Are you using a stock plug or running a hotter heat range?

 

Hard for me to judge as I replace my 125 piston at 25 hours intervals but personally your piston is showing more wear that I'd like to see in my engines.

Your specific off-road usage lugs an engine more than MX riding, putting more side loading on the piston.

Edited by mlatour
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