yz 125 shut down and wont start

alright guys, bit of a shitshow here.

Basically i was riding with some buddies last weekend and after we were riding for a couple hours, while going down a trail, my bike suddenly shut down, and since then i havent been able to start it again.

today we took off the carb and cleaned out the carb and the bowl, cleaned the reed valves, put it back together and tried to start it again but had no luck.

were probably going to look at the top end next, and i know this is quite vague but if anyone has some past experience or advice to share it would be much appreciated.

Thanks in advance,


Bad luck, has it a spark? And what color, does the spark plug looks wet or dry?

thanks for the reply arnego2,

changed the plug on the weekend, checked then and again now, has spark

Spark - Check. Verify that flywheel hasn't moved or spun so that timing is way off.

Fuel - ??

Compression - ??

You need all three for the bike to run. You can try a tablespoon of fuel in the spark plug hole to see if it will at least start for a second or two. Sudden shut-down doesn't sound like Carb - unless there is a problem with the float valve and no fuel is getting into the carb. Does the bowl fill with gas?

So I took apart the carb, cleaned all the jet passages with compressed air, and made sure the float was pivoting nicely, up and down. And the fuel is still over flowing from the carb when I turn on the fuel. When I removed the bowl, it was filled with fuel. We have compression, and spark. Fuel must be messed right???

So I took apart the carb, cleaned all the jet passages with compressed air, and made sure the float was pivoting nicely, up and down. And the fuel is still over flowing from the carb when I turn on the fuel. When I removed the bowl, it was filled with fuel. We have compression, and spark. Fuel must be messed right???

Sounds like fuel is at least getting to the carb Bowl. Overflowing is either float height not set correctly or float valve not working. If you take the float bowl and off and turn on the gas can you stop the flow by pushing up on the floats? If so then the height must be off so fix that. If not them the float valve might be work - pull out the neecle and check the rubber end piece for wear - should be smooth and a nice point on it.

You can still get spark even with a flywheel that has come loose or spun on teh shaft so check that as well (easy enough to do - just pull the cover off and look)

Try the Fuel in the spark plug hole trick. You can leave the fuel from the tank turned off. If spark is right and you have compression the bike should at least run for a couple seconds. If that happens then you are pretty much down to fuel delivery. Either not getting fuel or not getting the correct fuel / air mix. And blowing out the jets with air might help, but build-up of varnish inside the jets will cause major problems. I always opt for replaceing jets rather than trying to clean them.

Another way to check for proper spark, timing and compression is to shut off the gas at the tank. Remove the air cleaner and give a short shot of ether into the rear of the carb (one second only!). Give it a kick or two and she should fire up and run for a second or so. If it starts, you know you have a problem with the carb. If it doesn't start, your spark, timing or compression are now suspect.

DO NOT USE EXCESS AMOUNTS OF ETHER. You can do some serious damage to your engine if you spray too much (you can very quickly sieze it).

However, if it is flooding out just sitting there, it sounds as though either your floats are set incorrectly, or the float needle is bad. Double check everything inside the carb. I've had good luck cleaning jets with an old toothbrush and compressed air. Do not try to clean jets with wire brushes or pushing small pieces of wire through the jet opening.

Sounds like a carb issue to me...

okay bit of an update guys...ignore the fuel overflow problem, when i took apart the carb to clean it out, i stupidly forgot to soak the float in fuel so it was sticking, hence why the fuel was going straight out of the overflow.

once i put that back together, she started up first kick, although it does sound off...im thinking it might be a top end issue but im not sure, going to see if i can get a video or something up on here...a friend said it might be my piston rings?

How many hours does the top end have anyway?

Could be your piston rings but if you kicked it over time after time trying to get it started you may have flooded it. Give the bike a compression test and see whats up with the ring seal. If everything checks out with the top end Id go ride the piss out of it for a little and see if it gets any better, only after checking the top end out though

think i found the problem guys, powervalve actuating arm is busted...pretty sure this is what my problem is. If you guys can confirm that, that would be great. Thanks for all your responses.

Your bike should still start... unless the PV is stuck open I guess it could make starting difficult.

yes it does start. just sounds very different now. It wasnt starting because the float was sticking in the carb, but that has since been fixed.

If the PV is stuck in the open positon, then your bike will probably be down a lot on the bottom and mid-range. It will be a very pipey engine, even more so than it is normally.

You can run the bike like that, just know that it won't have a lot of low or mid range.

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