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2010 EVO 290 revved itself to death!


Big Timbob
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Evening all, I’ve somehow killed my Evo and its a bit of an odd one. 

Riding across a field with my boy when we stopped to open a gate. I stopped my engine with the lanyard, lent the bike against the wall, opened the gate and pushed my bike through before shutting the gate again. 
I kicked her over, the engine fired up then revved it’s t*ts off. I pulled the lanyard, then the plug lead, then turned the fuel off and it stopped. Leaving me little shaken and with a long uphill push home. 
Anyone have any suggestions? 

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Have you tried restarting it again? check that the throttle cable outer hasn't come out of the ferrule on the throttle or at the carb. You may find that's its all as it should be so you may not find anything wrong with the cable now, it may have re-fitted its self... if you have a tapered rubber grommet at the throttle, try putting a cable tie around both ends to secure it to the throttle housing and then to the outer throttle cable to stop the outer coming away from the ferrule on the throttle

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Gotta be a throttle cable pulled out as noted above. The only other possibility is a split open intake manifold or debris in the carb to hold the slide open. If it was a relatively short term event the bike may be fine. The damage to the owner is severe. If the bike runs OK there is most likely no damage to the top end as the chrome bores are pretty resistant until the piston actually starts fusion welding itself to the bore. If you still have normal compression your rings are not stuck (first clue of a seizure) and you probably dodged the bullet. Best to get into the habit of twisting the throttle before you start the bike and making sure there is a good solid thwock sound from the carb when you let go. Not only is it reassuring but it seats the cable.

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Thanks everyone for your input. 

I spent a couple of hours in the garage yesterday and so far I’ve not found the root cause. 

After the incident it felt like the kickstart was no longer connected to the crank so I was very concerned about severe engine damage, after removing the clutch covers I found the primary gear bolt was loose meaning the gear could spin freely on its taper (there’s no key). I don’t know if this was a result of the high revs or part of the cause (?) but I’ve torqued it back up and the kickstart now turns the engine over properly. 

I pulled the exhaust off and there is light scoring on the piston which I’ll investigate further when I pull the head and barrel off later this week. 

The carb seems to open and close fine - possibly a little slow but it was pulled out to one side. It’s nice and clean and doesn’t look to have been the cause. 

The reeds look fine, clean and crisp in closing so they don’t look to have caused it. 

Ive ordered a flywheel puller to see if it’s an ignition fault but that won’t arrive until mid week. 

Starting to run out of ideas for other candidates but I’m reluctant to buy a new piston (hopefully not a cylinder) only to restart it and it happen again. 

Might as well fit a new throttle cable while it’s in bits incase the old one is fraying internally. 

Surely this is an excuse to buy a newer bike? 

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You're doing the right thing. Only way to have peace of mind is to tear it apart and be certain but as thall says the most likely is the throttle cable and once it's reseated it's like a little kid saying, "What? Me! No I didn't do it.".

Ignition can't hold the throttle open and the engine won't rev without access to a lot of air so I'd not bother with that line of investigation.

One thing that seems counter intuitive when this happens is to grab the throttle and pin it to full for a split second. It often seats the cable allowing the engine to come back down. Of course when it's revving its nuts off that's the last thing that comes to mind but it often works. As for grabbing the spark wire I'll let the thing explode before I ever do that again. I'm rather fond of my heartbeat.

Edited by dan williams
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Good advice in this thread.  I've heard of several of these runaway engine situations.  Once the spark plug turns into a glow plug, hitting the kill switch or yanking the plug wire is ineffective.  Blocking the exhaust seems the only quick option.  Use your boot or some mud, a stick, a rock not your hand.  Nasty burn would result.  Blocking the intake wouldn't be as easy.

I know a guy who saved an expensive diesel engine that was running away by blocking the air intake with his jacket.

Big Timbob, do you remember if you leaned the biker against the wall on the right side or the left side?  Displaced throttle cable seems the most likely cause to me.

Edited by Sir Real Ed
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She’s back up and running!

New piston, rings, small end, reeds while I was in there, throttle cable after your advice on here and of course a woodruff key. 

Started second kick and ticked over nicely until my wife charged in and told me it was too late for that nonsense. ?

Hopefully she’ll be right for a bit now. 

Thsnks everyone!

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Good stuff! 

Mine did the same the other day. Bike lept away and went sky high on the revs. Even the lanyard didn’t stop it! Thumb up the pipe did. Eventually. Scary moment. I’ve not had it happen before. Throttle cable now cable tied in place! New woodruff key coming. I hope I’m lucky too and it lives to smash me up again!

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