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98 yz 125 won't run

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ok so i finally got my yz 125 together enough to take out for a test ride. i took it around the house a few times and then went down the road less than a quarter of a mile. when i got back to my lane it bogged really hard and shut off. while i was riding i noticed that it bogged too much on the low end. but now it won't start. i haven't touched it yet but it leaked a bunch of fuel out on the floor. it seems to have compression but i don't have a tester. I'm planing to get one soon. i did take the plug out and it looked fine, it was a light brown with no extra oil buildup so its not fouled. I'm guessing that it is something in the carb. i haven't had much time lately but hopefully i will dig into it this week. jw if anyone had any suggestions or ideas on what might be wrong.

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if it's leaking fuel out of the carb overflow then that means your float is improperly adjusted/sticking open, the needle valve or seat is damaged, or you have a cracked overflow tube. a sticking float can cause the bike to flood like you're experiencing.

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if it's leaking fuel out of the carb overflow then that means your float is improperly adjusted/sticking open, the needle valve or seat is damaged, or you have a cracked overflow tube. a sticking float can cause the bike to flood like you're experiencing.

yup what he said!

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hey thanks CRF100biker. that helps a lot. i will try that this week and see what happens. i expect it to fix the problem.

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ok so i had my carb out twice tonight and nothing. it wouldn't even fire. i adjusted the float and that stopped the overflow problem. i checked the plug after each time and both times it was wet with fuel and had shark. i tried a very simple compression test by putting my thumb over the plug whole and cranking it over a couple of times. it blew my thumb off but not with a lot of force. i just put a new top end in so i wouldn't think that it needs that. the cylinder seemed to be fine when had it apart. i read about a cold seizure. when the piston expands faster than the cylinder wall but doesn't completely lock it up. does anyone know the symptoms of this?

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have an air leak anywhere? tried putting a new plug in? sometimes twos just toast em, we had a perfectly jetted 91 rm125 for a while dad put a new plug in it every 3 rides and it ran like crap, stepped up to a hot plug and it ran perfect!.

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pretty much what your explaining... exact same symptoms but it will usually run after it cools back down. Im still leaning towards a carb issue if your plug is wet. Have you replaced the plug since you fixed the float?

If it was fouled the bike wont start until you get a new plug. Dont try to clean it because its pretty pointless and will likely just foul again. Just spend the $3 and grab a new one and get a spare while your at it.

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my freinds yz125 did the same thing pretty much. the plugs didnt look very fouled but there was erratic spark when you kicked it over. it would run with new pugs but kept fouling them. i put a hotter plug in it which helped before i realized that it was way too ritch in the low rpms. moved the needle down one clip and ran it, it was slightly better, then moved it down another and it ran perfect. your bike definatley sounds like the needle needs to be moved down and a hotter plug may help aswell. if your worried about the top end just take the pipe off and look at the piston through the exhaust port to see if its scratched up.

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hey guys thanks for all the input. i went to my local shop today and basically they said that i need a new piston. I'm new to two strokes and i didn't know that you can't take them on the road for the first couple of tanks. well i did and apparently that screws up my piston. i will get a compression tester and see what compression it has but i will probably end up getting a new piston. so thank you all for your help anyways.

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I'm new to two strokes and i didn't know that you can't take them on the road for the first couple of tanks.

i think this is a little excessive, it should only be a couple heat cycles then your bike should be broken in, not a couple of tanks, search around the forum here theres lots of articles on how to break in your new top end/

furthurmore, i would pay close attention to the carb, these couple years of the 125 for some reason seem to have carb issues develope, chances are if it has ok compression, with no visble scaring on the cylinder, the piston is probably fine, you may just need a new ring set if it wasn't broken in properly, and play around with float level/jetting, air/fuel screw. you could also possibly throw a hotter plug in it a as well, id also check the powervalves on it, ive had the pins rattle out of the powervalves renduring them useless. good luck

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well one thing that i failed to mention is that i lost a lot of compression. like i can crank it over with my hand very easily. and if it was only a carb issue then i think that is would at least do something. right now it doesn't even fire. the plug was wet when i checked it and it had spark so i would think that it would try to do something but it just cranks over and doesn't fire.

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He's still never answered my question as to whether he replaced the spark plug or just tried to clean it. If you just tried to clean it, replace it!

125's dont have much compression to begin with, I could turn mine over by hand fairly easy even with squish set/compression raised for race fuel. However a broken ring is still not out of the question and Im not telling him that this isnt his issue, but Id be looking toward weak spark, or timing issue based soley on the fuel and wet spark plug issues he's having. Eliminate the easy stuff before you tear the motor apart again.

FWIW: your dealer is full of it, you can run your bike however you'd like if its jetted properly, doesnt matter where... the road will do nasty things to your transmission over time however, or so Ive been told. I have a few VERY experienced mechanics, one of them being an ex Factory Yamaha mechanic that will tell you that they never broke in thier race motors (125s or 250s) other than starting them to make sure that everything was operating properly and getting them from the pits to the gate, and those riders rode thier bikes alot harder than we do. As his form of proof he told me that they would swap pistons between motos or right before practice and go out, warm the bike up and run the moto.

This isnt an attempt to prove one method is superior to another, as even I break my motors in, just not to the extent that some do. Just sharing a little knowledge that was shared with me.

Edited by ftball90

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hey guys thanks for all the input. i went to my local shop today and basically they said that i need a new piston. I'm new to two strokes and i didn't know that you can't take them on the road for the first couple of tanks. well i did and apparently that screws up my piston. i will get a compression tester and see what compression it has but i will probably end up getting a new piston. so thank you all for your help anyways.

Let me guess, they said you need a new piston and they fortunanetly have one on the shelf for 125$ and they'll only charge you 100$ to install it and professionally break in your fresh top end for you??:lol:

FYI, my break in consists of warming the bike up once, letting it cool completely. Warming it up a second time and then give it hell. And that is only for my sanity, most people won't even give it one heat cycle before giving it hell.

If you have fuel, you have spark and you have compression (if top end is in spec it should run with very very little compression) then check timing and jetting, and get a few extra plugs.

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Let me guess, they said you need a new piston and they fortunanetly have one on the shelf for 125$ and they'll only charge you 100$ to install it and professionally break in your fresh top end for you??:lol:

+1 on this, they're defiantly trying to bend you over the counter lol:smirk:

furthurmore, im a firm believer of the, warm it up, ride it hard, then ride it harder, method for breaking in:ride:

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Dude do not believe those shop rats. Just like all mechanics, they'll tell you anything to have you dump $800 at their shop when it could simply be the top of carb O-ring constantly letting too much fuel in and flooding. Check to make sure you are getting spark and check your timing. There are a couple threads going on right now with guys having your EXACT same problems.

Search titles: "wont start tried everything" and similar or actually just wont start will be perfect.

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well one thing that i failed to mention is that i lost a lot of compression. like i can crank it over with my hand very easily. and if it was only a carb issue then i think that is would at least do something. right now it doesn't even fire. the plug was wet when i checked it and it had spark so i would think that it would try to do something but it just cranks over and doesn't fire.

not necessarily i had this happen to our 125 when first bought it previous owner said it needed a new top end because it wouldn't run. well i tore it down to find everything looked ok i threw a need top end in anyways, went to fire it up...nothing same symptoms as your experiencing, wet plug, leaking fuel out of over flow, tried new plugs, had good spark nothing was happening, turned out the float height was wrong flooding the bike immediatly, but if your bikes due for a top end id do it anyways, but even with low compression if the fuel air mix is good it should fire/ it'll be gutless but still should run

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Yeah man to be honest my first thought after reading your original post was float bowl is too wide letting all that fuel in. In the other thread I mentioned that was the guys same problem as well.

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