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2008 yz125 build

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Decided to frame my yz125 and rebuild the motor. We have had the bike for almost 4 years now and love the bike. It has been very reliable with regular maintenance.

What's being done:
Cylinder porting (millennium technologies)
Hot rods crank assembly
Pro taper chain and sprockets
Pro taper handlebars
Updated ufo front fender
180 decals or magiksc graphics
Pro circuit expansion chamber

 

(sorry for the bad cell phone pics)

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Edited by Yamaha791

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We have over 200 hours on our 06, original crank is still perfect even with the extra load of the 144 kit.

I will never sell it.

Congrats on the 250F as well! I passed the 144 down to my son when I bought my 14, it will go to my daughter

after that.

Welcome to TT!

Edited by MJP68

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To Yamaha791,

since you are doing a complete engine refurb,

don't forget to check the combustion chamber's squish height and have it corrected if necessary.

 

Discuss it with the people who are doing the cylinder work.

Edited by mlatour

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To Yamaha791,

since you are doing a complete engine refurb,

don't forget to check the combustion chamber's squish height and have it corrected if necessary.

Discuss it with the people who are doing the cylinder work.

. Is that nessiary on a 125 . Edited by nicko-31

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Seems to be more of an issue on 250's but all 2-stroke can benefit from having the correct squish height.

Perhaps since 250cc or larger engine have a larger bore, thus making a greater surface area for pre-ignition / detonation to happen.

 

Having a too large squish height makes the engine more prone to detonation in the event of a batch of bad fuel / too low octane or too lean jetting.

Simply explained, at the top of the piston's stroke all of the air/fuel mixture must be squeezed out from around the edge of the bore (squish band)

back into the center of the combustion chamber.   If the squish height is too great, some of the mixture remains 'trapped' in that area,

when the squish height is reduced, the mixture gets squeezed out. With little to no air/fuel remaining in the squish band, there is less chance of detonation.

 

Factory tolerances on squish are often on the loose side, perhaps the reason stock bikes are

sometimes jetted a tad richer than ideal is to compensate for these possible issues.

It also raises the octane requirement in an otherwise stock engine to prevent or compensate for such issues.

 

This past summer I finaly decided to measure the squish in my '10 YZ125 and from the factory it came at a huge 1.35mm,

from what I've read a properly built 125 can safely run as low as 0.7mm - 0.8mm

 

As a 'safe' first attempt I reduced the height to about 1.1mm leaving plenty of material for further testing.

A simple before / after compression test showed a modest but appreciable increase of 5psi

 

I already mix about 25% race fuel so detonation was never a problem

but reducing the squish allowed for some slightly leaner jetting (needle's profile)

My YZ125 was already jetted to run very crisp, now with the reduced squish height it somehow runs even crisper.

 

For a bit more compression, a crisper running engine and lessen the chances of detonation, it's a 'free' mod that's well worth doing.

Edited by mlatour

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We have over 200 hours on our 06, original crank is still perfect even with the extra load of the 144 kit.

I will never sell it.

Congrats on the 250F as well! I passed the 144 down to my son when I bought my 14, it will go to my daughter

after that.

Welcome to TT!

That's great to hear about the crank durability.  I just bought an abused 06 for my 14 yo son, and I'm hoping the cranks is in good shape.  It runs great with no funny noises so fingers are crossed!

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In another thread I've read that Shout Spot Remover may work as an aluminium cleaner.

 

Otherwise, if the frame isn't too bad, Mother's Mag & Aluminium Polish works very well, mirror finish with little effort.

Edited by mlatour

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Eagle one Nevr dull and your favorite radio station.... I had a lot of luck with the Mothers powerball? The drill attachment with the Mag and alluminum polish. If you have the frame stripped all the way down, soda or sand blasting should be cheap and quick at a powder coater.

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Soda blasting...

X-2 ,You'll be amazed how well it comes out and won't harm delicate parts such as seals.

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I always clean my frame with sos pads. If I get bored in the winter I use mothers for more shine.

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Seems to be more of an issue on 250's but all 2-stroke can benefit from having the correct squish height.

Perhaps since 250cc or larger engine have a larger bore, thus making a greater surface area for pre-ignition / detonation to happen.

 

Having a too large squish height makes the engine more prone to detonation in the event of a batch of bad fuel / too low octane or too lean jetting.

Simply explained, at the top of the piston's stroke all of the air/fuel mixture must be squeezed out from around the edge of the bore (squish band)

back into the center of the combustion chamber.   If the squish height is too great, some of the mixture remains 'trapped' in that area,

when the squish height is reduced, the mixture gets squeezed out. With little to no air/fuel remaining in the squish band, there is less chance of detonation.

 

Factory tolerances on squish are often on the loose side, perhaps the reason stock bikes are

sometimes jetted a tad richer than ideal is to compensate for these possible issues.

It also raises the octane requirement in an otherwise stock engine to prevent or compensate for such issues.

 

This past summer I finaly decided to measure the squish in my '10 YZ125 and from the factory it came at a huge 1.35mm,

from what I've read a properly built 125 can safely run as low as 0.7mm - 0.8mm

 

As a 'safe' first attempt I reduced the height to about 1.1mm leaving plenty of material for further testing.

A simple before / after compression test showed a modest but appreciable increase of 5psi

 

I already mix about 25% race fuel so detonation was never a problem

but reducing the squish allowed for some slightly leaner jetting (needle's profile)

My YZ125 was already jetted to run very crisp, now with the reduced squish height it somehow runs even crisper.

 

For a bit more compression, a crisper running engine and lessen the chances of detonation, it's a 'free' mod that's well worth doing.

 

This all seems to be the opposite of what I have learned about squish... unless I am reading this wrong.

 

Too large squish doesnt cause detonation... its too small a squish band. Due to it being impossible to keep tolderances exact enough in mass production engines come out with larger squishes as a 'safe zone'.

 

If your squish is near 1.4mm then youre wasting money on high octane gas if you ask me... assuming your compession isnt through the roof which I cant see why it would be. A builder will machine to get the proper squish for a motor... like .9mm etc then machine the dome after that so that compression is within spec.

 

Also, why would you ajust your squish little by little like that? What's the point? Why not just set it at .9mm and then make sure the dome is proper cc's?

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Let me add to that explanation YZ144FirstTimer, see in bold below :

 

quoting myself,

If the squish height is too great, some of the mixture remains 'trapped' in that area,

the trapped mixture is then super compressed to the point of self-ignition or detonation

when the squish height is reduced, the mixture gets squeezed out and into the center dome part of the head.

Since the space is reduced, with little to no air/fuel remaining in the squish band, there is less chance of detonation.

 

I quickly checked mine as an evening project with the engine still in the frame, just removing the head.

Next time it's due for a full dis-assembly & top end, I will look more closely at setting the height and the cc's in the combustion chamber.

 

Also the reason I prefered to reduce mine at a smaller increment and test is that I did not want to increase overall compression

to the point of having to increase my fuel's octane requirement.

Edited by mlatour

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