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WOT sputter

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Hello fellas,

I have a 2003 yz125 that begins cutting out (sputtering NOT bogging) but losing a ton of power none the less at 3/4 plus rpms. Low and mid range arent too bad, the powerband never really hits as hard as i feel it should. I ride at about 6000', top end is fresh, reeds are fresh, power valve is working properly and the carb is clean with a 410 main, 40 primary, stock needle set in the third position and the air screw at 2 turns out. The spark plug is alittle more towards the black side than brown, so i know its a tad rich. Floats are also set correctly. The pipe is stock and the silencer is also stock and freshly re packed. Im very frustrated at this point and am starting to lean towards either the CDI box or a coil being the problem. PLEASE help. I dont want to miss out on a whole season of riding.

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Is a #410 main jet the stock size for that model year?

If so it might be too rich for yout 6000' elevation.

 

Also, what octane gasoline are you running?

Higher elevation decreases the octane requirement.

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Im not 100% if these are stock jetting sizes. I got the jetting sizes off one of the forums. I run 91 octaine fuel mixed at 32:1 ratio. Screenshot_20181008-202805_Chrome.jpeg

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Pro Circuit has this as stock specs: 

2003 YZ125

air screw: 2 turns out

pilot jet #40

needle #6CHY6-32 3rd position

main jet #410

 

Has the bike ever been ridden in the same conditions before and, previously ran better?

      

I've never experienced it but from many posts here from people who regularly ride at higher elevations,

due to the lower air density the engine loose power and it's relatively common to compensate by:

 

-reduce the squish height / bump compression

-run lower octane gasoline 

-jet leaner

 

Other members will hopefully chime in with some tips

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Searching your posting history, your powervalve linkage was damaged and

you were looking for an alternate fix rather than buying the OEM replacement part. 

 

How did you end up fixing it?

Can you confirm the mechanism (not you by hand) opens the valves properly and doesn't bind.

Edited by mlatour

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Haha. Im impressed! Yes, this bike had a ton of issues when purchased. To fix this problem, a bought good used complete power valve assembly. When putting these in, i tried the powervalve mod found in a thumper talk discussion and installed an actuator from a 1999 yz125 (heavier spring). I have since put the stock actuator back in to see if this was the problem. (It did not fix the problem). I only know the powervale and linkage is working by moving it by hand. Im not sure how to test it while the bike is runing.

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I also think you are a bit too rich on the main. Try a 400 to see if improves for your elevation. Try a better plug also , your existing may be breaking down at high revs.

I also have a 2003 125, I am only a few hundred feet elevation but run the settings below which are working well for MX

410 main

37.5 pilot

1.5 turns out on AScrew

2004 powervalve governor spring model

Std needle at clip position 2

50% Shell VPower Nitro pump 50% Sunoco surge leaded 105 - 32:1 Motul800.

NGK BR9EIX plug.

 

Edited by TommyCrown

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Okay, i will pick up a new Main jet and spark plug and give these a shot. What about CDI boxes or coils? Do either of these items effect a bike at high rpm's. The sputtering rather than the bogging makes me feel like the problem might be on the ignition side vs. Fuel. Btw, how do you check the power vlave operation while the bike is running?

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1 hour ago, Gibson41741 said:

how do you check the power vlave operation while the bike is running?

You could remove the little cover and with the engine running (on stand)

observe if the linkage moves the same amount when revved out as when you operate it by hand.

 

Part #29

https://www.partspitstop.com/oemparts/a/yam/50044d38f8700209bc791201/cylinder-head

Edited by mlatour

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What was/will be the air temperature at when riding?

Likely requires a drop of 2-3 steps on the main

1/2-1 step leaner on needle(from stock position)

check your pilot(likely rich) and effects all jetting ranges, per standard testing procedures

octane level is fine

consider having Eric Gorr cut your head for high altitude riding to get back some of the compression your losing due to decreased atmospheric pressure.

It could be a bad coil, but try the jetting first as your rich @6000 ft

 

Edited by mrdsee

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I appreciate all your suggestions so far fellas, however i have solved the mystery of the sputter. I went through the jetting process and wound up dripping the Main from a 410 to a 400. I dropped the pilot jet from a 40 to a 37.5 and left the clip position in the number 3 postition with the air screw at 1 & 3/4 turns out. I put a new spark plug in it, a BR9EIX and ran a chop tesr. Porcalyn looks like a light mocha. See picture. I also took off the PV side cover and varified its operation. As best i could tell, It looked to be operating properly; however i did see the linkage bounce a little durring the sputter condition. I checked the timing and found it set within spec. I am throwing this post out one last time before heading to a professional. I am more than frustrated with the bike at this point. I took a short video of the condition to give a better perspective. Disregard the awkward take off, i had my phone in my clutch hand and was more concerned with dropping it while letting out the clutch. It sputters badly in secoong gear. Condition exists in all geara though. Thank you in advance for all your help fellas. Hers a link to the video.

20181207_151221.jpeg20181207_151211.jpeg

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Found a critical error in my last post. I meant to that i have NOT solved the mystery of the sputter. It seems to happen just about the time the power band should be kicking in.

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Watching your video, the first thing for me that springs to mind is ignition based on what I'm hearing. I would look to make sure that the coil and stator windings are in spec and that you have good ground connections.

Trim your coil to plug-cap wire, and maybe get a new plug-cap to be sure.

Could be the CDI, but i think the only test is to swap it out.

Has your timing been adjusted at all?

Are your reeds in good shape?

Yooj

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Check your coil, cap, etc etc per service manual testing procedures. Unplug all electrical connections(even rotate kill switch a little) and reinsert using di-electric grease. Keep us posted on results.

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Also put the needle clip in the #2 (move up 1, 1 step leaner) slot and retest.

Mid range too rich

Edited by mrdsee

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Also found this:

"Yamaha for 01-03 lightened the Pv spring rate, this causes the PV to open to soon and can create a bog like feeling or dip early in the powerband.

For 04 Yamaha went back in time to the 00 PV spring and solved this problem."

 

I changed the spring on my 125 for better results. So you might want to explore this further.

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