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Spun rotor !

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Recently performed a CDI conversion on my 90 XR 100 after getting frustrated to no end with it running poorly. The CDI did the trick thats for sure !

Anyways during the install I decided to use the stepped woodruff key on the crank. The one that was shipped to me didn't fit exactly perfectly into the slot on the crank but i was able to position it on the crank and put the rotor over it.

To make a long story short i was out riding it the other day at low speed (fortunately) and the bike began to sputter and made one or two knocks and died and would not restart.

Tonight I removed the valve cover and confirmed that the valves were moving freely and the piston spins fine. When i removed the side cover i was in for a huge surprise as i realized the TDC center mark on the rotor was 180 degrees out from actual top dead center for the piston and the 0 mark on the cam gear.

I am a little puzzled as to how this occurred, I removed the rotor and the key appeared to have been sheared in half down the middle horizontally !! My only thought is that the nut on the rotor was not tight enough argh.

My question is, should i expect any internal damage ?

From a valve/piston perspective the timing appears to be fine obviously from an ignition perspective not so much so :) I dont feel any wobbling or anything strange with the crank shaft but the shaft is scratched up now where the rotor mounts on shaft. My concern would this cause any sort of catastrophic detonation or anything else evil that I might be missing ? :lol:

thanks

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Had a mismatched cam sprocket on a ttr125, apparently they changed tooth count one year and the dealer didn't check. Anyways got it back together and the TDC kept changing in respect to valve timing. Tried to start it and the valves ended up kissing the top of a brand new piston. Tore it down again and ended up getting the right part finally from the dealer. Didn't see any damage to the valves, but we ended up changing them just to be on the safe side anyway. There were some small marks from the exhaust valve (I think) in the top of the piston but we just sanded them out smooth. Bike runs fine now and that was over a year ago. I would at least take the head off and have a look if it were my bike. Good luck.

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Only two functions for the flywheel, neither of which involve valve timing; flywheel weight for the engine and magnets for the alternator which powers the ignition.

Ignition timing is controlled by the points or CDI sensor so a slipping flywheel won't change that but its output could become out of phase with ignition firing and the engine would lose spark.

The key is designed to locate the flywheel on the crank for assembly not transmit torque, that is the job of the taper. So your idea that the bolt wasn't tight enough seems likely. The taper needs to be clamped tight enough so it doesn't slip. Two things can help; clean the taper with a solvent to remove any oil residue, and apply proper torque on the bolt.

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Whenever, we used to spin a flywheel in the past, we would lap them in so the taper would be a cleaner fit. Also, we would impact on the flywheel, then run for about 15 minutes and then impact it again.

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Yeah no reason to tear it down. Don't know what I was thinking lol. Brain fade.

Easy because the symptoms you described of how the bike ran could be from altered ignition timing which on some bikes and most older cars is done from the cam indicating a possible valve timing problem.

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So I called a few of the usual suspects (engines only etc.) and no one seems to stock the stepped woodruff keys any longer :bonk:

anyone aware of any other sources for the keys by chance ?

if not i will run it keyless i suppose. My only concern is keeping the flywheel completely lined up with the slot on the crank when tightening. I will be using an electric impact gun most likely. any suggestions on how to keep things lined up without the key !?

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You don't need a stepped key just go with a standard key, make it fit. And as others said clean up the tapers and lap in with a lapping compound for a good fit. Clean everything up and bolt it together ( I like carb cleaner).

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I used to run them with a stock skinny key to line it up with. Just line it up by sight, then tap it gently with a socket, them impact it. You'll know if it moved a bit by how it runs. What's nice is you can advance it a bit if you want.

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