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C50_img_2742_1504803287 mcarlisle Quote
12/11/2016 11:06 AM

Alright, so I'm not the most mechanically inclined but I somewhat know my way around 2 strokes. I have a 2002 yz125 that will start and run pretty crisp for about 1-2 minutes on the stand, then I put in first, take off and before I can even make it a lap around the track it bogs out bad and just dies. Won't start back up after that unless I give it a few minutes and tip it over and let all fuel drain out of the overflow. I have jetted it back to stock as I have no idea what the guy before me jetted it at. I'm at about 200ft elevation here so stock shouldn't be too off even with it being around 50 degrees here now. Air screw on carb is stock at 2.5 turns out. Just replaced reeds. Carb is spotless and blown out ever inch with air. Stock everything on this bike basically at this point. Gas mixture is 40-1 with 927 and pump gas. Gas regularly comes out of overflow even if the bike has been sitting a while so don't know if the float is actually working like it should or not. Don't thing the float is sticking though. Anyone have any ideas? Power valves maybe? I also happened to be filming when this issue started so I'll put the link to the video. The video shows right when the bike started running real bad. Just seconds before the video the bike sounded and felt good and crisp. So I already know the bike sounds like crap in the video.

C50_img_2742_1504803287 mcarlisle Quote
12/11/2016 11:29 AM

And here's a link to another video of the bike dying again after changing spark plug and tipping bike over to drain fuel from carb.

C50 Bruce372 Quote
12/11/2016 11:54 AM

Unscrew the gas cap and try again. Sometimes the vent can block

C50 Bruce372 Quote
12/11/2016 11:55 AM

Probably needs a float adjustment aND a new float needle seat and seak

C50_chappelle_02_0205_act2_1472771519 project racer Quote
12/11/2016 1:06 PM

Sounds like it is starving for fuel. Like Bruce mentioned, check the fuel cap for venting. Does the petcock flow freely, clean? Just noticed the check valve in the vent tube on the gas cap. Is it installed correctly? Is it flowing air in the proper direction ? Also check the vent tubes on the carb, make sure they are not blocked / pinched.

C50_img_2742_1504803287 mcarlisle Quote
12/11/2016 1:21 PM

I've adjusted the tab on the float a couple times now and I've gone pretty far with it with no changes. I started to wonder about the gas cap since it will be sitting on the stand and I can sometimes hear a gurgling sound coming from it like it's releaseing the pressure in the tank. As far as the petcock, it flows gas like it should when open and even with it closed it still has a small flow of gas. So it doesn't actually stop the flow of fuel completely. Don't know if that is normal. I really appreciate the ideas guys. Keep them coming and I'm going to double check all those things.

C50_img_2742_1504803287 mcarlisle Quote
12/11/2016 1:24 PM

Forgot to mention this. The cover to the power valves had one of the screws back out all the way and was just hanging there. There has been a good bit of oil/sploog coming from it. So I thought maybe it was sucking air so tightened that best I could but not as tight as it should of course bc I was not able to take the motor out trackside. Can the power valves give you issues like this?

C50 Paw Paw 271 Quote
12/11/2016 1:44 PM

What you are hearing when you take the fuel tank cap off is air rushing into the tank ( not pressure in the tank, but rather a vacuum) as the cap vent is not working and when it is not working it will restrict of even shut down the fuel flow.
Along with that I suspect that your carb vent lines may be clogged. No air = no fuel flow.
Reset the float level to factory spec's

Paw Paw

C50_img_2742_1504803287 mcarlisle Quote
12/11/2016 2:09 PM

Paw paw, I don't really hear anything when I take the fuel cap off. Just hear it when the bike is on the stand. Should I maybe get rid of the plastic piece on the hose? Sorry don't know correct terminology for that little thing. I will check the carb vent lines and blow them all out again though because I could have missed something there. So if it wasn't getting fuel wouldn't there not be any fuel coming out of overflow?

C50 Paw Paw 271 Quote
12/11/2016 3:54 PM
"I started to wonder about the gas cap since it will be sitting on the stand and I can sometimes hear a gurgling sound coming from it like it's releaseing the pressure in the tank."

That was what you said.
That plastic part is the tank vent. It lets air into the tank, but does not allow fuel to spill out. They go bad.

Paw Paw
C50_100821080_1287530718 kzizok Quote
12/11/2016 4:55 PM


C50_funny_beards_11 HardTaco Quote
12/12/2016 10:26 AM

Cdi if your tank and carb vents are working correctly. I've been through this and it was my cdi all along.

C50_116925730_1228927739 TJ 755 Quote
12/13/2016 8:53 AM

stators will quit when they get warm and then cool back down and work. Usually you need to get it pretty hot like after a couple laps but I have seen it both ways.

C50_img_2742_1504803287 mcarlisle Quote
12/13/2016 5:02 PM

Ok when it comes to electronics I'm not so good but I have messed with a stator on a street bike before. I know you can test a stator, I'm assuming you can test the cdi as well?

C50_1983_8_1478968938 speed_racer Quote
12/14/2016 7:55 AM

We need spark, fuel, air, compression to run.
One of these is disappearing at some point.

My guess is wet plug, he gets it to run again with plug and unflooding/leaning bike.
Probably not compression, but easy to check in 48 seconds. It should blow you finger off the hole when kicking.
Fuel? Lean? I would ride with choke on and fuel cap almost loose and see if it keeps running without stalling.
Rich? Flooding? This is probably the culprit because a new plug helps it start again? Is the plug wet?

Proper diagnosis happens before a single part is replaced, you must find out what is missing when it dies,
fuel, air, spark, keep it simple.
Keep a spare plug in your pocket, when it dies pull over and pull plug cap, put extra plug in cap and kick bike over to see if the spark stopped when the stator got hot, takes 20 seconds.
If the bike is flooding then holding the throttle wide open while kicking might give you a sign, if it keeps trying to start with the throttle wide open then that is showing the engine is fuel flooded.
My guess is that you are losing spark because the plug is so wet from fuel it won't fire.
Maybe the float has a pinhole, see if it floats in water? Shake the float and listen for fluid inside.

Good luck

C50_img_2742_1504803287 mcarlisle Quote
12/14/2016 2:59 PM

Thanks for the replies. I really do appreciate the input gents. As for the plug, when I took it out it looked a little wet but not too bad. I've pulled way worse out when it was running like a top and just running rich. I'm sure that doesn't still mean it's not too wet now though. It still has compression but I feel like it's not much compression. But since this is the first 125 I've had since I was 14 years old back in my racing days it's kind of hard to remember or compare just from kicking it over. I will try the plug in the pocket idea. Again thanks for the input from everyone. I'll finally be able to really put some time in this weekend to figure it out. I'll get back with results.

C50_618115028_1203301146 xnationalwrench Quote
12/15/2016 12:21 PM

You might try disconnecting the kill button.

C50_20160812_120930_1472074274 656mx Quote
12/16/2016 6:12 AM

I had a 96 kx 125 do the same thing once. It ended up being a crank bearing . It never siezed and would start back up a few minutes later. Like others have said check for spark and fuel right after it dies.

C50_chadreed2016_copy_1491167139 theonlinelegend Quote
12/16/2016 8:24 PM

I had something similar once. I had just swapped the reeds to carbon fiber ones. And I must have installed them incorrectly. What does yours look like?
And man, knowing that thing is going to cut out or die and still jumping it. Not this guy.

C50_img_2742_1504803287 mcarlisle Quote
12/16/2016 8:46 PM

theonlinelegend haha yeah I thought that right after. Didn't know it would though until it did it the second time. So needless to say I just went in circles on flat ground for a bit. I have boyesen power reeds in it.

C50_img_2742_1504803287 mcarlisle Quote
12/28/2016 12:45 PM

So went through the carb again. Blew the whole thing out and took it all apart. I ended up replacing the needle valve seat for the float. The o-ring must have been no good anymore because the old one set in with no resistance and even had a small amount of wiggle room. The new one was so tight that the retaining screw actually pushes it into its spot. So I think fuel was just bypassing it before even when the float was shutting off fuel. No more fuel coming out of the overflow and now bike starts first kick even cold. The bike is still a little rich but that's a simple jetting thing. Thanks for everybody's help!

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