YZ 125 custom engine mods

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3/30/2020 6:49 PM
Edited Date/Time: 3/30/2020 7:03 PM

Hello Everyone,
I have 2012 YZ125 with maybe 70-100 hours on the chassis. I want to build a bike for a GNCC race- more bottom end response smile
Bottom and top end freshly rebuilt. Engine cases are filled with epoxy and putty. Reeds cage inlet is filled with putty to reduce crankcase voulume. I plugged the oil holes of the main bearings and made side grooves to provide premix to the bearings. Cylinder ports are cleaned, exhaust port is polished, polished cylinder head, cases and cylinder are matched (crank case with epoxy). I have swapped a carburetor to Keihin 38mm air striker from 2008 rm250. The real dimension is 38.79mm. Squish is 1.5mm. There is Vforce, and Pro Circut system with shorty. (bike came with it but I think I should look for different system designed for bush ridding) Sprockets 13-51. Spark plub BR9IX but will go to BR8EG (hotter one)

I took it for a first ride this weekend. Jetting: needle clip 3rd, main 172, pilot 45, 1,25 air screw.

Unfortunately there is no bottom end on the bike. Power delivery is wired. No bottom end, and over half way throttle there is like switch ON, and the bike goes like crazy! It is not smooth. Bike has so much power, revs to the moon, doing uphills you can shift gear up. Really fast.

I am still looking for a more bottom end. First I will go back to the stock carb to see it 0.8mm make such a big difference. Next step will be trying 36 mm Keihin carb- my buddy has one. Then heavier fly wheel. Maybe I will try thicker cylinder gasket, Finally shaving cylinder or cylinder head (probably cylinder to drop exhaust ports)- doing this only if I will gain bottom end. Shaving cylinder head brings higher compression and more top end. I want to try it with the stock reeds also.

Chances of failure are high but it might work smile





What do you think?


3/31/2020 11:46 AM

Heavier fly wheel might work helping on the lower rpm range, but never forget that a 125 have quite limited power. Have you tried to dyno it?


3/31/2020 1:33 PM
Edited Date/Time: 3/31/2020 1:34 PM

Heavier flywheel won't help, all they do is reduce the hit. A 125 was never ment to have great bottom end power. Did you do the porting? It could be that it's ported for top end power rather than bottom end
Also, a DEP torque pipe and shorty silencer will help bottom end


3/31/2020 4:35 PM

I wouldn't have been plugging those holes for the lube for the main bearings....


3/31/2020 10:08 PM

SXS, no I haven't put it on the dyno yet. But when I feel the bike is good for sure I will do it!

chumpt6784, Cylinder is not ported, I only cleaned it from the cast unperfections and match transfer ports with the crankcase. Didn't remove any material. Yes, different pipe for sure will be a way to go! But first I need to deal with carb settings, different carb etc. Also I talked with my buddy and he suggested to play with timing. I totally forgot about that and I will do it on the next ride!

I will try to contact Bill's Pipes to build me an off-road pipe for my bike. Anybody done that?

PTshox, sometimes it is fun to risk smile If the will seize I will put it a part, drill holes and replace bearings.




4/1/2020 7:09 AM

Odd that you have no bottom end power after all you did. I have always found the Keihin to make more torque than the Mikuni. With putty in the intake, it should make a little more torque. a 45 pilot sounds a little lean on a 125 but just a guess. Interesting build!


4/9/2020 7:07 PM
Edited Date/Time: 4/10/2020 11:44 AM

Cut your cylinder base .5mm. That will lower the port timing and pull the power peak down a little lower in the rpm range. It will also tighten up the squish to 1mm. You can open the chamber in the head to put back some volume lost with the tighter squish, but increasing the compression will boost the low end.

You did a lot of grinding in the boost port, you did raise the port or increase the angle? That may be your lose of bottom end.

As mentioned the Keihin normally makes a little better low end power than the Mikuni. I assume you're using the NECJ needle that comes stock in the RM carb. You might try the NECW needle, it's a step richer on the diameter.

You shouldn't need the 8 plug if you are racing GNCC type stuff, only if you are planning to do hard enduro. Same with the FWW.

You'll be drilling the main bearing oiling holes back out shortly.


4/9/2020 10:05 PM

It looks like a temporary plug in the mains lube holes, in the assembly photo they look clear.


4/9/2020 10:58 PM

Are you sure the power valve is working properly? I have a 07 and it has a healthy low end both bone stock and with full yrrd tuning kit. I ride enduro with 13-48 gearing and it works fine.


4/10/2020 2:56 AM

Umm did no one read his 1.5mm squish?? Could pretty much halve that!


4/10/2020 8:26 AM

Yeah, it's way wrong. Needs to be about half that.


4/10/2020 10:12 AM

From my experience Robinson nailed it. U need to lower the ports. Don’t run two base gaskets as this will raise them and make more top end.


4/11/2020 6:19 PM
Edited Date/Time: 4/11/2020 9:40 PM

Mr. Robinson wrote:

Cut your cylinder base .5mm. That will lower the port timing and pull the power peak down a little lower in the rpm range. It ...more

You are right, I have grind the boost port. I have made it as Eric Gorr's cylinder. Maybe it is touch too much (If future changes will not help it- Epoxy will do so) I didn't do any changes to the heights or angles.
This makes sens that I haven't gain bottom end...
With the needle do you think I should go richer not leaner? For sure I will try both options but to me it seems the other way.


Banana_oil- engine is for sure put together right.
nthebadboycorner it is JB Weld
I will double check the squish- there is a single gasket from Athena gasket kit- it is thinner then OEM...

I have just gotten a 36 mm Kehin carb from 2013 TE 300. Softer exhaust spring is in the way! Will instal it and will see what is the next step:
a) drop exhaust ports by milling the cylinder and keep 38 mm carb
b) Rise the compression by milling cylinder head and keep 36mm carb

How low can I go with squish to be able to stay with pump fuel? 0.7-1mm?


2/18/2021 4:23 AM

Any updates mxFloga (sorry that's how I read it the while time till I just checked spelling 😁)


2/18/2021 4:51 AM

I ran 0.9mm squish with no problems.


2/20/2021 2:58 PM

It’s pretty cool of you to wing it, get in there and try stuff.
I recently cleaned up the casting on my cylinder and did a little polishing on the exhaust side, I didn’t notice a big difference, but it was fun to give it a go


2/21/2021 5:40 AM

Out of curiosity what’s the potential upside to plugging the oil holes?


RPM Performance

2/21/2021 7:22 AM

I think the 38mm carb is a big part of your problem


3/21/2021 7:43 AM

Hello everyone!

Thank you for being interested in my project. Unfortunately, things are not going as planned…
I have installed 36mm Keihin Carb. First kick and bike sounds like Factory 125 KTM which I saw at World Enduro Championships a few years ago. Super crisp!!! I spent some time to perfectly jest the carb. Bike was awesome. 5th gear and it was doing wheelies with out the clutch. Sitting position with only only fork pre-load and cracking the throttle. I took it it the Enduro classic test track. BIKE WAS FLYING. Was pulling from the bottom, reving high, and was pulling hard. Surprisley, it was not flat on top. Bike was puling great and reving high. It was lacking some torque, because when I was traversing it on a difficult terrain it was hard to get traction. (It is stil only 125). Doing tight corners- PERFECT.

I took the bike to the Offroad sandy area, pretty much wild motocross track in the woods. Deep sand whoops all the time. Houston we got problem—>
I was just warming up and hit whoops section like I used to. I was 4th gear, going pretty fast, leaned backwards and cracked the throttle. I tried to wheelie the whole section but… Bike vent to vertical and too fast. No time to tap the break or ditch the bike. I landed on my butt and L1 exploded into pieces. I am paralyzed waist down…

I don’t know if it was not time to get used to the bike, or power delivery was uncontrolled (I loved it in the bush). I am wondering how the bike would be with heavier fly wheel… I didn’t have chance to put it on dyno. I flew to Europe to the rehab centre. Put bike back to stock and going to sell it. Very upset because I was very excited about the project and invested lots of time into it… I didn't have time to change cylinder base gasket to get squish around 1mm, modify cylinder head, get heavier flywheel...

If you are interested in my story, check out the Bent Methods podcast hosting me. If you can support me rehab process, please buy a t-shirt.

Go Found Me:

See you on a trail one day!!!


3/21/2021 7:45 AM

murph783 wrote:

Out of curiosity what’s the potential upside to plugging the oil holes?

I think you can seized crankshaft bearings or connection rod bearing. I think...


3/21/2021 1:07 PM

Damn I’m very sorry to read this. I hope you make a full recovery.. this sport is brutal


3/21/2021 3:39 PM

chump6784 wrote:

Heavier flywheel won't help, all they do is reduce the hit. A 125 was never ment to have great bottom end power. Did you do ...more

Pro Circuit put a heavier flywheel on RV's bike; PC is including a heavier flywheel with it's YZ 125 RV spec kit..



3/21/2021 3:43 PM

mxFOLGA - I hope you get better.



3/21/2021 8:02 PM
Edited Date/Time: 3/21/2021 8:03 PM

Sorry to hear about your injury.

Just curious what are you doing for therapy?

I work with SCI pts and a couple of them have the RT300 stim bike, which works great for keeping the legs going with blood flow, maintain muscle and cardio.

I also do water therapy with one of them.

Also, do you use a standing frame?


3/22/2021 11:23 AM

Sorry to hear about your injury. Hope you make a full recovery and fast!


2017 RMZ450
2005 YZ250-sold :,(
1998 YZ250
2005 KX250F

80% of the time it works every time
IG @2HRacing
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