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Guys,
HELP. 2008 YZ 125. Sounds crisp until you hold it wide then it breaks up. I rebuilt carb with stock jets. Bike is stock. Put in new crank seals, new top end. TILL happening. Carb has all new float, jets, seat etc.. Measured fuel level. My gut tells me MAYBE reed or coil??? Coil ohm to spec. PLEASE any help would be appreciated.
Old piston looked good. Idling high even with throttle cable out.
Thanks
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Did you check the reeds for cracks or proper seal?
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1998 YZ250
2005 KX250F
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I've seen a few late model YZ125's with a very small air leak where the reed block meets the cases. A bit of Hondabond cured that issue on my son's 05.
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Just to be clear, are you revving the bike in neutral on a stand or under a load going through the gears?
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Matt,
How did you find the leak on your son?
Sandman,
Breaks up either on the stand or when being ridden.
Pulled the reeds last night. Excessive gas in the intake area.
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So I am going to do a leak down to check. Quick question, is it best to do the test inputting air through the plug hole or intake?
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go with a fatter main jet than stock and see how it runs. Are you running any race gas?
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Location: GBR
When I leak down tester mine I pressurised through the plug hole. Whereas all the diy testers were through the intake. Doesn't make any difference to be fair.
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Don't overlook coil or possibly the spark plug cap is open circuit. Ohms values are in manual. These faults will cause this symptom.
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MXRICH wrote:
Matt,
How did you find the leak on your son?
Sandman,
Breaks up either on the stand or when being ridden.
Pulled the reeds last night. Excessive gas in the intake area.
I made my own pressure tester with Home Depot plumbing/gas parts. Used a rubber freeze plug in the exhaust, a big bolt in the powervalve overflow line. Pumped it up to about 6 psi and started spraying soapy water around all the gasket spots. Eventually found a very minor leak where the reed cage bolted to the cylinder. It was so minor that when the bike was cold it didn't leak, but once warmed up parts would expand enough that the leak would create a lean bog, particularly under a load.
Spraying ether, carb cleaner, WD40, etc at the running engine had no duplicatable effect, though that method has worked for me in the past.
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Matt,
That is my plan on Friday. My understanding is the power valve shouldn't pass air so is it necessary to block the vent tube?
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From what I remember (it's been a few years since I needed to test that bike) it leaked into the PV area. I think it also leaks from around the PV cover on the right side where the cylinder and case meet, where there's that odd shaped gasket (small air leaks in these areas are post-combustion and so don't affect how the bike runs). Not a fast leak, but enough that you'll likely need to keep slowly adding air while spraying the soapy water. I used a bicycle pump since an air compressor would be too easy to blow something out (even the gaskets themselves).
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Joined: 2/21/2013
Location: Hillsborough, NJ USA
Update: rigged an air source to do leak down. seems like all air is out through power valve.Plug the hose but seems to bypass seal. Pressure drops right down. Make sense that power valve seal would do this?
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Yes, it's going to be normal for some pressure to be lost through the PV system. It has a vent hose, the rectangular cover above the exhaust port, and the covers/gasket where the actuating arm goes to the PV itself. Since it's after the combustion it's not going to affect the air/fuel ratio. For pressure testing, block them off as best you can.
I was able to slowly keep pressure with the bicycle tire pump, just enough to find the air leak. Much easier with 2 guys, one to pump and watch the pressure gauge, another to spray and look for bubbles.
Also, check for pressure escaping from the transmission vent tube. If this is occurring, you either have a bad right side crank seal or the center cases have a leak.
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Matt,
It i escaping through the seal for the power valve shaft. I was forward thinking and ordered them along with crank seals. Arrived today. Will replace in the am. The air pressure bleeds off immediately. seal behind flywheel is good. Frustrating as hell. head gasket, intake all look good. Anyway to to tell if clutch side seal is leaking WITHOUT pulling basket?
THANK YOU!!
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Take your transmission vent line and shove it in a bottle of soapy water. That way any leaks from the right crank seal will bubble for you and you're not just listening for air to come out.
Posts: 64
Joined: 2/21/2013
Location: Hillsborough, NJ USA
Update: Looks like it was the power valve shaft seal. Replaced it. Friday and yesterday at Budds my son was hammering it. No break up. During practice yesterday it broke a little. 1/2 turn on air screw cleared that.
THANK YOU ALL!!
