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Road Race XR100 vibration only at idle

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My 2003 stock 100 XR100 road racer has developed a vibration only at idle. Really weird and I'm sure it has something to do with me jamming down two gears and revving the pants off of it coming out of the back straight at top speed.

I find it really odd though that I don't feel it anywhere other than idle. I would expect it to get worse as the RPMs increase. It starts normal. It idles fine (other than the vibration), it runs like normal.

 

Any suggestions on where to look first? The season is almost over and I'm going to finish on this motor unless someone is 95% certain it's about to throw a rod or something that will completely trash the motor is going to happen in the next, say five hours of running.

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Make sure all the motor mount bolts are tight and that there are no breaks in the frame.  While you're at it make sure the swing arm bolt is tight and check the steering head bearings for play also.  And, pull the left side cover and flywheel (look over flywheel, taper and key) ............ and check crankshaft run out (wobble at the end).  

 

Old School Al

Edited by Old School Al

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It might just be a loose part on the bike, such as the muffler heat shield or a piece of plastic. It could also be a loose cam chain.

 

My XR100 vibrates at WOT. Its a pretty distinct point when it will vibrate (about 8,500 RPM). Haha I use this vibration to know when to shift (I shift at about 9,500 RPM)

 

I don't think your bike will throw a rod or something unless you've got a ton of mods. These motors do not blow, they just wear out (after many, many hours).

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Would you be surprised to hear that I never had another opportunity to go racing this year? I either had the time and didn't have the money or I had the money and not the time or twice, I had the time and the money and the weather was cold so I decided that it was best to spend the day on the couch instead of spending the day skipping across the pavement because the track is cold and the tires don't adhere well to a cold track which makes wipeouts more common.

 

I'll get back to you when I find something.

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I was thinking about this thread, I'm guessing it's time for an update.

So I never did get to investigate this. I've since been to two miniGP days and would you believe the vibration hasn't come back? I before the first event, no chunks, no slivers, no bright metallic sheen, no sign of anything amiss. I'm not going to tear down a functioning motor for shits and giggles. However, I'm getting a false neutral between 4th and 5th about twice per day (let's call it every 50 laps) when this gets a little more frequent, I'll swap that motor out with another one I've bought with the intent to build up, but haven't yet. I may tear it down, but no promises if I'll put it back together, most likely I'll keep it for spare parts or cores unless there's a visually obvious cause for the false neutral.

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