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YZ125 Over Rev

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I'm currently racing motocross with a 2010 YZ125. Boyesen Rad valve, 13/49 gearing, PC R304 silencer, 400MJ/35PJ/6CHY5-80 3rd position. I'm looking for a little more "oomph" to allow me to hang onto 3rd and 4th gear a little longer. I like the way the bike runs with the stock pipe, but will change it if I can find something that won't give up too much bottom end. Any setup recommendations?

Edited by MxWrench91

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First would be getting rid of that R304, shorty silencers are detrimental to good top end / overev. 

The stock 2010 silencer is pretty much the optimal length for best overall power.

 

Another free mod would be to experiment retarding the ignition timing about 1 degree at a time, late timing favors top end & overev.

 

I also ran 13/49 for a long time in mine but as my skills improved 

found the stock 13/48 avoided running out of gear/power too soon and requiring a misplaced upshift right before a jump.

 

Usually a YZ250 issue but measuring & correcting the squish band compression height is well worth the trouble,

Mine went from a huge 1.37mm (stock) down to 1.05mm which made the engine run yet even crisper and more tolerant to 

a wider spread of ambiant temperature changes before requiring any re-jetting. It gained a modest 5psi of compression as well.

 

I'm not familiar with that needle, I'll have to look it up.

Edited by mlatour
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The 2010 service manual doesn't show your 6CHY5-80 needle so for now I can't compare to anything I'm familiar with

but the -80 suggest it has a larger diameter meaning it should be leaner than stock.

 

stock 2010 (north american) specs are: 

#40 pilot jet

#6BFY42-74 - 3rd clip

#410 main jet 

 

myself run:

#40 pilot (ran a #35 and #37.5 for a long time to prevent spooging but it's missing some out on out-of-corner grunt when running too lean)

#6BFY43-74 - 2nd clip  (meaning a 1/2 clip leaner than stock, my engine responds best with this needle)

#420 main jet   (#410 on tighter tracks or when not riding in deep sandy/soft soils)

 

32:1 premix,  91 pump premium with 25% race fuel for a total of about 95-96 octane.

Stock pipe & silencer, Boyesen Pro Series reeds, OEM reed stoppers.

Edited by mlatour

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52 minutes ago, MxWrench91 said:

I'm currently racing motocross with a 2010 YZ125. Boyesen Rad valve, 13/49 gearing, PC R304 silencer, 400MJ/35PJ/6CHY5-80 3rd position. I'm looking for a little more "oomph" to allow me to hang onto 3rd and 4th gear a little longer. I like the way the bike runs with the stock pipe, but will change it if I can find something that won't give up too much bottom end. Any setup recommendations?

50T rear sprocket 

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28 minutes ago, mlatour said:

First would be getting rid of that R304, shorty silencers are detrimental to good top end / overev. 

The stock 2010 silencer is pretty much the optimal length for best overall power.

 

Another free mod would be to experiment retarding the ignition timing about 1 degree at a time, late timing favors top end & overev.

 

I also ran 13/49 for a long time in mine but as my skills improved 

found the stock 13/48 avoided running out of gear/power too soon and requiring a misplaced upshift right before a jump.

 

Usually a YZ250 issue but measuring & correcting the squish band compression height is well worth the trouble,

Mine went from a huge 1.37mm (stock) down to 1.05mm which made the engine run yet even crisper and more tolerant to 

a wider spread of ambiant temperature changes before requiring any re-jetting. It gained a modest 5psi of compression as well.

 

I'm not familiar with that needle, I'll have to look it up.

Sorry, my fault. I am running the 6BFY42-74 needle. I have been considering adjusting my squish myself. Did you happen to cc your head before cutting and did you alter your combustion angles? I'd like to try and avoid race gas. 

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+1 on retarding ignition timing. 2 degrees retarded will give you huge overrev. If that loses too much low end oomph, try 1 degree. 

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12 minutes ago, MxWrench91 said:

Sorry, my fault. I am running the 6BFY42-74 needle. I have been considering adjusting my squish myself. Did you happen to cc your head before cutting and did you alter your combustion angles? I'd like to try and avoid race gas. 

No unfortunately I didn't measure the before and after cc's, I left my dome is stock / un-corrected,

no exact data but from what I have read and calculated, it probably reduced the combustion chamber's cc's by less than 1cc.

 

I was preventatively running my race gas mix before I reduced the squish height,

detonation wasn't a problem but rather as 'insurance' in the event of a bad batch of pump gas.

 

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I'll try the silencer switch, retard the timing 1 degree and double check my jetting to get me started. I'll cut the head during the winter. Thanks for the info!

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Like you, next season I'm looking forward to experiment retarding the timing to gain a bit more overev.

I'm willing to trade off a bit of low end as sometimes it's hard to get traction coming out of corners on a particularly tight track I commonly ride at.

 

The factory ignition timing specs for the 2010 YZ125 is set with the piston at 0.48mm (0.019") before top dead center.

I do have a dial indicator but not a degree wheel.

 

It's getting pretty late this evening to get my mathematical and geometry skills going but

for those who have participate to this thread, how can we establish a degree of flywheel rotation into a piston position measurement?

(example, each 0.05mm equals 1 degree)

 

Thank you in advance.

 

Edited by mlatour

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27 minutes ago, mlatour said:

Like you, next season I'm looking forward to experiment retarding the timing to gain a bit more overev.

I'm willing to trade off a bit of low end as sometimes it's hard to get traction coming out of corners on a particularly tight track I commonly ride at.

 

The factory ignition timing specs for the 2010 YZ125 is set with the piston at 0.48mm (0.019") before top dead center.

I do have a dial indicator but not a degree wheel.

 

It's getting pretty late this evening to get my mathematical and geometry skills going but

for those who have participate to this thread, how can we establish a degree of flywheel rotation into a piston position measurement?

(example, each 0.05mm equals 1 degree)

 

Thank you in advance.

 

Not sure if this helps what you are looking for. It is a screen shot I saved out of the YZ250 timing thread. I think it was about halfway through the thread.

YZ125 Timing.PNG

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Thanks for the link GettingThere, :thumbsup:

gives me a good starting point of what to expect.

 

Ideally I'll get a flywheel degree wheel/tool and compare with my dial indicator.

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6 hours ago, mlatour said:

Thanks for the link GettingThere, :thumbsup:

gives me a good starting point of what to expect.

 

Ideally I'll get a flywheel degree wheel/tool and compare with my dial indicator.

No problem. Here is a link to the thread and the settings for the 125 were on page 12. A post from Oct. 30, 2009. Keep us posted what you figure out.

 

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I got around to taking some measurements this evening. My current squish is 1.40mm and ignition timing 0.024" BTDC, putting me at 1 degree advanced. Stator plate marks still aligned from the factory, I've never had the stator off the bike. Readjusted my timing to 0.015" (1 degree ret.) I'm not sure if I'll get the chance to try it this weekend. I'd like to cut the head over the winter to get my squish down to 1.00mm and recut the combustion chamber to keep me in the pump gas range. I'll try to CC the head this weekend if I get the time.

Edited by MxWrench91
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Measured mine a few days ago, ignition timing had never been touched from factory settings.

Everything was spot-on specs,  0.48mm (0.019") BTDC with the timing marks aligned.

 

If you get to ride this weekend I'll be looking forward to your impressions of the 1° later test.

 

Myself done MX'ing for the season and maybe get in another trail ride if weather permits.

(don't mind riding when cold, just hate washing the bike and equipment in freezing weather)

 

Edited by mlatour

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23 minutes ago, mlatour said:

Measured mine a few days ago, ignition timing had never been touched from factory settings.

Everything was spot-on specs,  0.48mm (0.019") BTDC with the timing marks aligned.

 

If you get to ride this weekend I'll be looking forward to your impressions of the 1° later test.

 

Myself done MX'ing for the season and maybe get in another trail ride if weather permits.

(don't mind riding when cold, just hate washing the bike and equipment in freezing weather)

 

Technically 2° if I'm going from 1° adv (.024") to 1° ret (.015") from stock (.019"). I threw my degree wheel on and at a quick glance it looks like .015" is 7° BTDC. I'll have to bend up a better pointer tomorrow to confirm.

44429118_1973948866231841_8147194386577358848_n.jpg

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I have a PC pipe with a stock silencer currently on my 03 (previous generation motor).  While I don't think it does much for overall power, I specifically like it for woods racing as it does allow me to hold a gear a little longer with higher revs.  It definitely has better over rev than my 06 which is currently sporting the stock pipe and silencer.  Not sure if the new motor would respond the same way with a PC pipe.

As others have discussed, gearing is a big issue.  In general, I think people gear down too much, which makes you shift more.  Get yourself a couple of cheap sprockets break out the stopwatch at your favorite track.  Remember that the butt dyno is a pathological liar.  Stopwatch always tells the truth.  

Call me skeptical on the timing mods.  Better check with Mr Stopwatch.  

Edited by rpt50

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Has anyone measured their combustion chamber volume before? I only had a syringe available and measured 9.2cc with the spark plug installed.

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