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YZ125 runaway engine

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Have a 2015 YZ125.  After a rather bad crash I had to replace the subframe and air box.  I put it all back together. Kicked for 5 minutes  Wouldn't even come close to starting.  Had been sitting for 6 weeks or so, so I checked and Got spark, got compression, got new plug(old one showed proper color, not lean), got gas supply to carb. Anyway I lay the bike on its left side for a highly technical excess fuel purge and first kick with fuel off and zero throttle, it starts but have runaway engine condition.  Had to use kill switch to stop it before it blew up. I have been reading that it could be crank seals or another type of air leak.  Not really buying that because why would it start right up after simply laying it on its side.  New air box should be sound and sealed.  Anyway I would think the air leak if it's that is after the carburetor.  On the YZ there is only the reed valve after the carb.  I'm thinking crud in the carb but on a dirt bike you would think that would be a more common condition.   Anyway, looking for ideas here. 

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Dude do a leak down test. It will tell you if there's a leak, and then have soapy water ready to look for bubbles

Dude do a leak down test. It will tell you if there's a leak, and then have soapy water ready to look for bubbles

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Yeah make sure you check to see your slide moves up and down, and also make sure your carb is getting enough gas

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Pinched and/or stretched throttle cable's outer sheathing as well.

 

Happened to me a few rides ago after a minor tumble where the RH grip got buried deep in soft dirt.

The cable's outer sheath got stretched where it enters the throttle assembly and made for an intermittent 'hang-up' / WOT.

Edited by mlatour
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There was a guy on here a while back who had runaway as well, turns out his gas tank was empty. I think what we're trying to get at is check the simple stuff first, don't go pulling your engine apart yet lol. Sometimes it's an easy fix that you may have overlooked

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Throttle cable and slide moves smoothly and freely.  Slide was at definitely at bottom of travel.   Got plenty of fuel flow from tank to carb fuel inlet. 

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Ok, how does the carb boot look? Any visible cracks or anything? Is it firmly installed so no air will leak in between the boot and the cylinder mounting surface?

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On 18/08/2017 at 7:06 PM, ASP1227 said:

Ok, how does the carb boot look? Any visible cracks or anything? Is it firmly installed so no air will leak in between the boot and the cylinder mounting surface?

You probably mean the carb intake manifold. At least I classify the airboot as the rubber throat connecting the airbox and the carb. Correct me if I'm wrong, though.

 

Have you tried choke on, half throttle, full throttle kicks?

I'm sorry if I missed it, but have you inspected the carb itself? All jets clean and no dirt inside whatsoever? A nice looking spark plug doesn't necessarily mean a proper A/F ratio, in a case like this at least.

 

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