YZ139 Max Power RPM's Kit

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4/4/2020 8:39 PM
Edited Date/Time: 4/4/2020 8:41 PM

I had worn out my stock cylinder on my 2014 yz125 and had needed to repair some detonation from the previous owner on the top corner of the cylinder. I decided to go through Max Power RPM's due to their good reputation. After talking on the phone for a few minutes they had sold me on their 139 kit.
4 weeks and a lot of great service and communication later, I received my bored and ported cylinder, machined head and power valves all assembled. They provided very detailed instructions and break in procedure along with a gasket kit and piston kit. Very professional and clean, they have 10/10 service.
After breaking in the motor and getting my lectron tuned for the cylinder it runs great! I was very worried about it having little to no overrev like 144's are known for. The 139 has more torque and hp in the low to mid range than stock, and the overrev is damn near identical to the incredible 125 overrev. It still likes to be screamed like a 125, but it can also be ridden a little more lazy if desired. The double ringed wiseco piston weighed slightly less than the stock piston, so despite the bore being bigger, very little overrev was lost. I had it ported for top end power and had the head cut for 100 octane fuel and I run klotz R50 at 30:1. I have a vortex ignition, and vforce reeds, and an fmf shorty on the bike as well that really make the bottom end punchy and responsive.
This is truly the whole package. Strong off the bottom, powerful midrange, and 125 like overrev. I fully recommend this kit, it feels like it could definitely be competitive with a 250F. I found that the power comes on the best with the stock pipe and shorty muffler. The stock pipe has a great spread and the shorty really makes it punchy out of the corners. I can't say enough good things about this kit and I definitely plan on sending more cylinders to MaxRpm's in the future!
All of my additional parts and most importantly the 139 kit created a mean 2 stroke combo that is hard to beat!

https://maxrpms.net/category/motorcycle-kits/yamaha/yz125/

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4/4/2020 9:19 PM

I had a 139 done by them a few years back. I tested different exhaust setups back to back and completely agree with you in the fact that the shorty silencers seem to compliment the 139 big bore power delivery. It was deceptively fast and seemed to really improve my corner exit speed.

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4/4/2020 9:45 PM

b_kowalsk wrote:

I had a 139 done by them a few years back. I tested different exhaust setups back to back and completely agree with you in the fact that the shorty silencers seem to compliment the 139 big bore power delivery. It was deceptively fast and seemed to really improve my corner exit speed.

The 139 really is the bread and butter for the 6 speed yz engines! Any bigger and I noticed the overrev drastically dropped. What pipes did you experiment and have good luck with? I tried a few different fmf pipes but none of them felt like they transitioned from the bottom to mid range as well as the stock pipe.

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4/4/2020 9:57 PM

b_kowalsk wrote:

I had a 139 done by them a few years back. I tested different exhaust setups back to back and completely agree with you in the fact that the shorty silencers seem to compliment the 139 big bore power delivery. It was deceptively fast and seemed to really improve my corner exit speed.

Ter12 wrote:

The 139 really is the bread and butter for the 6 speed yz engines! Any bigger and I noticed the overrev drastically dropped. What pipes did you experiment and have good luck with? I tried a few different fmf pipes but none of them felt like they transitioned from the bottom to mid range as well as the stock pipe.

Try an hgs. In all my reading related to yz pipes hgs is the only one to really improve over stock without taking away from another area.

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4/4/2020 10:10 PM

Are you sold on the Lectron carb, as well? Worth the $$?

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4/4/2020 10:25 PM

Awesome topic here gents.

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Non Gratum Anus Rodentum

4/4/2020 10:57 PM

FWYT wrote:

Are you sold on the Lectron carb, as well? Worth the $$?

Yessir it most certainly is! I have the 38mm HV and it's an awesome product. Normally with big bores it is endless tinkering and jetting to get it running right, but with the lectron I leaned it out 1/2 turn from base settings (since the big bore is sucking more fuel and air) and set the power jet to 1 1/8 turns out. Took me 5 minutes get this bike dialed and I haven't had to touch it. Badass product!!! And it is very consistent in all elevation and temperature. I have over 100 hours on this carb and i'm never going back

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4/4/2020 11:01 PM

FWYT wrote:

Are you sold on the Lectron carb, as well? Worth the $$?

Ter12 wrote:

Yessir it most certainly is! I have the 38mm HV and it's an awesome product. Normally with big bores it is endless tinkering and jetting to get it running right, but with the lectron I leaned it out 1/2 turn from base settings (since the big bore is sucking more fuel and air) and set the power jet to 1 1/8 turns out. Took me 5 minutes get this bike dialed and I haven't had to touch it. Badass product!!! And it is very consistent in all elevation and temperature. I have over 100 hours on this carb and i'm never going back

Thank you!

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8/7/2020 1:45 AM

I know I'm a couple months late to all this.

I'm running 13/50 gearing on the 125. If I go up to the 139, would it work better stock gearing at 13/48?

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8/8/2020 9:10 AM

Hgs makes 144 pipes, dep is a good choice too for a big bore...

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8/8/2020 3:35 PM

theluckyb wrote:

I know I'm a couple months late to all this.

I'm running 13/50 gearing on the 125. If I go up to the 139, would it work better stock gearing at 13/48?

13/48 is what I’ve been running. I even tried 14 48 and it pulled it pretty well. I was surprised.

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8/8/2020 3:36 PM

suzukifa2014 wrote:

Hgs makes 144 pipes, dep is a good choice too for a big bore...

I’ll take a look at HGS and DEP. It looks like they both offer 144 pipes

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8/8/2020 3:52 PM

FWYT wrote:

Are you sold on the Lectron carb, as well? Worth the $$?

Ter12 wrote:

Yessir it most certainly is! I have the 38mm HV and it's an awesome product. Normally with big bores it is endless tinkering and jetting to get it running right, but with the lectron I leaned it out 1/2 turn from base settings (since the big bore is sucking more fuel and air) and set the power jet to 1 1/8 turns out. Took me 5 minutes get this bike dialed and I haven't had to touch it. Badass product!!! And it is very consistent in all elevation and temperature. I have over 100 hours on this carb and i'm never going back

I had one on my last 2 stroke. A 1980 YZ250G with the Answer Products "Bob Hannah port job" and I agree very responsive and easy to tune even for a 15 yo kid

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