So I have this 2006 KLX110 that I worked on for a friend. The top end was completely worn out when I got it so I put on a 143cc big bore kit and replace both valves. The bike ran great and he took it home. His kids rode it for about two weeks and it suddenly started making this noise so he brought it back to me. I feared the crank was failing so I pulled the top end back apart but the crank feels good and I did not see anything wrong in the top end; however, when I pulled the cam chain tensioner it was fully extended so I thought it was timing chain slap. I installed a new timing chain and the tensioner felt normal going back together however, the noise is still there. I've pulled both side covers off of the engine and don't see anything wrong in either side. Valves are in spec. The noise is still there with the clutch in and the transmission in gear so that eliminates any transmission parts. The noise isn't a steady, repetitive sound so I don't think it is anything to do with the crank/rod/piston. I pulled the clutch back apart and it looks good. The cam sprocket is tight and feels good. I have a mechanics stethoscope but the noise seems to be everywhere. Maybe a little louder in the bottom end but that can be because the case acts like an amplifier. So I'm stumped and don't know where to go from here. I still think it sounds like timing chain noise but I don't see anything wrong in there. It's not a heavy knock like a rod but a lighter sound almost like a rattle.

Anyone have any suggestions?

Here is a video so you can hear the sound.

 

 

 

Yes, the engine eventually seized and I had to replace the crank. I used a stroker crank which was cheaper than OEM so now it is a 155cc and sounds good. I'm not sure why the sound was so inconsistent, I've never heard a rod knock that sounded like that but that seems to be what it was.