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The Best YZ125 Mods That Make A Difference [MX or Enduro]

Want to make your YZ125 faster? Whether you’re looking to build the fastest 125 2 stroke or just want a powerful and light trail bike, this guide will cover the top 7 YZ125 mods you can do that will actually make a difference. 

I’ve owned several YZ125’s over the years, and there’s a lot of upgrades available for this motocross bike, but they don’t all have the same effect when it comes to an actual improvement.

2003 Yamaha YZ125 23 The Best YZ125 Mods That Make A Difference [MX or Enduro]
2003 YZ125 W/ 134cc big bore

Best Bang For Your Buck Mod: Jetting

The single best mod for most 2 strokes, including the YZ125, is getting the jetting right. You can easily gain torque, horsepower and throttle response in minutes, even if your bike seems to be running well. 

This 2 stroke jetting guide will show you how to start getting results in minutes, even if you’re new to jetting. 

JD Jetting kit vs DIY

Do you want to know how your bike works and save money, or do you just want a bolt-on part that will probably make it better? While the JD Jetting Jet Kit (Amazon) is a good option because it has custom needles for low and high elevation, it’s pretty expensive if you just need a jet or two.

The main difference between jetting the carb yourself and the JD kit is that if you know what jets you need, it’s much cheaper to buy them individually. If you ride at different elevations or climate, the JD kit may be an advantage due to the custom needles – the stock needle is pretty good and is adjustable.

Suspension upgrade

Once you get your YZ 125 running strong and reliable, the next best thing to do is get the suspension dialed in for your weight.

The spring rates need to be correct for your weight and type of riding. See this spring calculator for an estimated number. Stock dirt bikes are usually set-up for 160-180lb riders. 

How to make your YZ125 handle well

If you have the correct springs in, you need to get the bike balanced to handle properly. This is done by setting the ride height. 

Adjusting the shock sag is the first thing you should do to set the ride height. When the rear end is too low, you’ll have more stability, but turning will be really difficult because it’s harder to get weight on the front-end, and vise-versa if the rear is too high.

The forks can be adjusted by moving them up or down in the triple clamps, as well as adjusting the preload with an added spacer. 

Clickers before re-valve?

Adjusting the compression and rebound clickers are the next step to tuning. Try turning the clicker in 5 clicks at a time until you notice a difference. Trial and error until you find the most comfortable setting for you. 

When should you send your suspension out to get re-valved?

Only after you get the right springs, set the sag and adjust the clickers should you even consider paying hundreds or thousands of dollars to get a re-valved shock and forks. Not only will this potentially save you a lot of money, but you’ll have a better understanding of what you like and don’t like about your current suspension setup.

2004 Yamaha YZ125 rolling chassis with pro action suspension
Pro-Action tuned YZ125 suspension

For example, if you have the right springs and the compression is still too soft after turning the clickers in because you’re doing big jumps, you’ll be able to tell your tuner that.

Unfortunately, too many people make the mistake of just shipping the suspension out and saying “Make it better”. But if you don’t know what exactly you want better, then your tuner is going to have a hard to doing that, and then you’ll be unhappy because you spent $2000 and it’s worse than stock…

You may also find out that most stock dirt bikes have really good suspension once you spend a little bit of time tuning it to your liking.

YZ125 Big Bore Kit Worth It?

Going from 124cc to 144cc is a 16% increase in displacement. You can expect that much of a power gain with proper jetting. 

Slight case mods are required, but these can be done without splitting the cases if you’re careful and take necessary precautions. 

Going with the Athena 144 big bore kit (Motosport) for your YZ125 allows you to do everything yourself, which allows you to save a few bucks and you know what’s actually going on in your engine.

YZ125 Athena Big Bore Kit The Best YZ125 Mods That Make A Difference [MX or Enduro]

On the other hand, if you don’t have the time or confidence to do the engine work, you can send it out to a reputable builder, such as Eric Gorr or Kustom Kraft.

Porting – what it actually does

Cylinder porting is where you can make or break the power-band on a 2 stroke engine. The YZ125 has pretty good ports from the factory, but there’s always room for improvement. 

If you don’t know what you’re doing, I strongly warn you NOT to port the cylinder yourself. It’s very easy to make it run worse. Going bigger with the ports is not always better. In fact, sometimes smaller in some spots is better for overall performance. 

Find someone that knows the YZ125 engine and can give you the type of power-curve you want. This could be a low-end boost in torque and throttle response for woods riding, shifting the power towards the top-end and over-rev for a screamer, or somewhere in between. 

YZ125 Reeds upgrade

Stock reeds do a good job, but they’re nothing special when it comes to performance. These are the two most popular reed cage upgrades to get better power and throttle response:

Boyesen Rad Valve

Boyesen updated their Rad Valve (Motosport) to a one-piece aluminum design that’s super simple and easy to install. When you update the jetting (it usually runs richer with new reeds because it pulls in more fuel), you will notice more power and better response throughout the RPM range.

Boyesen Rad Valve The Best YZ125 Mods That Make A Difference [MX or Enduro]

VForce 3 Reed Cage

The Moto Tassinari VForce 3 reeds (Motosport) have been developed to give you sharper throttle response and more over rev. Their new design is more efficient, lasts longer, and provides more air which increases power. 

VForce 3 Reed Valve System The Best YZ125 Mods That Make A Difference [MX or Enduro]

It’s also fairly easy to install. If you can remove the carburetor from your YZ125, you’re halfway to changing the reed cage, which is just a few more small bolts. 

One small disadvantage compared to the new Boyesen Rad valve, is that the VForce requires two gaskets to seal it, giving you a higher chance of an air leak. With that said, if the surfaces are clean and the gaskets are good, you’re not likely to ever have a problem.

Boyesen power wing – who’s it for?

Want a little more low-end torque and throttle response? The Boyesen power wing (Motosport) is one of the easiest YZ125 mods, but it’s not for everyone.

Boyesen Power Wing The Best YZ125 Mods That Make A Difference [MX or Enduro]

It increases the air velocity at 0-1/2 throttle openings, so it’s best if you like to lug or short-shift and ride in the low to midrange of the RPM. If you’re a racer and ride wide open where a YZ 125 works best, this mod is not for you.

It also may require a slight jetting change to the air screw or needle position for the best throttle response and performance.

The most important mod if you upgrade the reeds…

Whether you get a VForce or Boyesen reed cage, you’re not going to see a performance gain unless you adjust the jetting accordingly. You’ll most likely have to go leaner on the jetting, but every bike and set up is a different.

YZ125 Pipe upgrade – which is best for you?

The pipe, next to porting, is the biggest factor to determining the power curve. 

Two of the most popular pipes for a Yamaha YZ125 are the FMF Factory Fatty (Motosport) and the Pro Circuit Works (Motosport) pipe. 

FMF Factory Fatty Pipe The Best YZ125 Mods That Make A Difference [MX or Enduro]

Kris Keefer tested both of these with a respective shorty silencer from FMF and Pro Circuit. 

His results after riding the bike back to back with each exhaust system came back with the FMF setup being slightly more upper-rpm and over-rev biased, while the pro Circuit was a little better on the low-end and mostly mid-range. 

Scalvini factory works pipe

Want something different and a little more “trick” look compared to the traditional FMF and PC pipes? The Scalvini factory works pipe (Motosport) gives your YZ125 build that custom look, along with a better sounding and performing exhaust.

Scalvini Factory Works Pipe The Best YZ125 Mods That Make A Difference [MX or Enduro]
Scalvini – a true “cone pipe” works look

With that said, the quality fit & finish isn’t always as good as the top name brand exhaust companies, so just be prepared in case it doesn’t fit “perfectly” the first time you install it.

Choosing the best pipe for your type of riding

Based on those results you choose what you want for power characteristics. I would choose the FMF Factory Fatty and FMF shorty (Motosport) for big and fast motocross tracks, as well as open desert riding. 

The Pro Circuit Works Pipe would be better for a beginner or if you want to lug your bike more and short-shift. 

Pro Circuit Works Pipe The Best YZ125 Mods That Make A Difference [MX or Enduro]

YZ125 Cylinder head upgrade

Increasing compression is one of the easiest ways to gain power, throttle response, as well as easier jetting!

An aftermarket cylinder head will give you more torque and better throttle response, as well as make jetting easier. This is done by correcting the squish clearance between the top of the piston and the head. 

A stock YZ125 has a loose squish tolerance, which is simply due to Yamaha manufacturing it on the “safe” side. Too little clearance will require race gas or engine failure. 

The VHM and Phathead are the most popular aftermarket cylinder heads for 2 strokes. They both offer a variety of combustion chamber inserts for differing compression ratios. You can set it up to run on pump gas or race gas

Best YZ125 Enduro Mods

Whether you have a YZ125 or YZ125X enduro model and race off-road or trail ride, there are a few mods to make it more durable and reliable. These are the best YZ 125 enduro mods for woods and off-road riding:

Radiator braces

Feel like you’re going to fall and twist your radiators? A solid set of Flo Motorsports braces (Motosport) are top of the line because they’re not only stronger than typical braces, but they offer more air flow to keep your engine running cooler.

So, if you’re enduro riding with lots of rocks or logs, having braces can help prevent a race or day of ending from falling on the radiator and causing a leak.

Flo Motorsports High Flow Radiator Braces The Best YZ125 Mods That Make A Difference [MX or Enduro]

Caution: Install radiator braces when your YZ125 is new or before you fall. Once you twist or tweak the radiators, aftermarket braces won’t fit properly or help prevent bending a second time.

Clutch upgrade

Whether you like to abuse the clutch or just need to because it’s a 125cc 2-stroke, upgrading the clutch will help make it last longer without slipping or overheating as easily. For the best clutch upgrades, Rekluse now offers a manual and auto-clutch kit.

The Rekluse Core Manual TorqDrive (Motosport) is an expensive upgrade, but it’s definitely a mod to consider if the stock YZ125 just isn’t cutting it. Not only will the TorqDrive last longer, but you’ll have better feel while slipping the clutch coming out of the corners, which is essential to ride the 125 fast.

In addition, somehow Rekluse managed to make the clutch lever pull slightly easier than stock, even though it’s stronger. This will help on those long rides or endurance races when the clutch gets hard to pull in at the end when your arms and hands are dead tired.

Rekluse Core Manual TorqDrive The Best YZ125 Mods That Make A Difference [MX or Enduro]
Rekluse Core Manual TorqDrive

Willing to spend a little bit more to get an automatic clutch with better performance? Even if you don’t like using the clutch to add power but don’t want to worry about stalling your YZ 125, the Rekluse RadiusCX Clutch Kit (Motosport) is their latest offering.

You still get the same lever feel as stock (or better/softer), but the auto clutch works for you instead. It automatically slips the clutch and keeps the engine running.

You can tune the engagement so that it feels like a stock clutch that engages almost instantly, but it’s so efficient that your clutch discs will likely last longer than if you were manually slipping the clutch.

Rekluse RadiusCX Clutch Kit The Best YZ125 Mods That Make A Difference [MX or Enduro]
Rekluse RadiusCX Clutch Kit

Is an aftermarket water pump worth it?

Is your YZ125 overheating often? Installing a Boyesen Supercooler water pump kit (Motosport) can help, Jimmy Lewis from DirtBikeTest has tested these with little success. As a result, it only reduced the coolant temp by a few degrees.

Boyesen Supercooler Water Pump Kit Blue The Best YZ125 Mods That Make A Difference [MX or Enduro]

You see, when you increase water flow in the cooling system, it pushes through the radiators so fast that it doesn’t get enough time to coolant it down. So, you’d actually need to upgrade the radiators as well to get a more noticeable improvement in cooling.

You’d be better off adding a quality fan kit (Motosport) to your radiators. They pull more air through, which is so important when you’re riding slow, which is common if you’re trail riding or doing hard enduro type trails.

Trail Tech Universal Fan Kit The Best YZ125 Mods That Make A Difference [MX or Enduro]

Two free mods to prevent overheating

First, make sure your clutch is properly adjusted. If it’s partially disengaged, your clutch is going to be slipping all the time, which will cause it to overheat much quicker.

The second free mod is improving your clutch control. It’s a common mistake to abuse your clutch by slipping it too much by reducing the power to prevent stalling, but that only causes your clutch (and engine) to get hotter much quicker and boil over.

Pipe guard – which kind is best?

Just bought a pretty new pipe for your YZ 125 but don’t want it to get smashed up? Depending on the pipe guard you get, it can prevent big dents or lots of little dents that make your pipe look ugly and lose power.

Eline offers trick-looking carbon fiber pipe guards (Motosport) that are strong and protect the most common area of your pipe that gets dented. It’s a solid piece and fairly durable, but it can still shatter if you hit it hard enough on a rock or big log.

Moose E Line Pipe Guard FMF 2 Stroke Pipe The Best YZ125 Mods That Make A Difference [MX or Enduro]

I would personally choose a different route, depending on my budget.

The Moose flexible pipe guard (Motosport) is what I have used because I’m cheap. It’s not going to prevent your pipe being smashed from a big hit, but it will do a good job against small rocks and roost that put tons of small dents into it.

Moose 2 Stroke Pipe Guard The Best YZ125 Mods That Make A Difference [MX or Enduro]

Bottom line: Best YZ125 Mods

The more mods you put together that are meant to improve a certain area of the powercurve, the better result you will get. But none if it will matter much if you don’t learn how to jet your YZ125.

The best performance mods and upgrades for your YZ125 are:

  • Jetting
  • Suspension tuning
  • Big Bore
  • Porting
  • Reeds
  • Pipe
  • Cylinder Head
  • Radiator braces
  • Clutch upgrade
  • Pipe guard

Best mod for trail riding

Suspension, jetting and gearing are important to make your YZ125 comfortable to ride in the woods and off-road, but that’s just the beginning. You need to learn how to ride properly. Clutch control and balance are fundamental techniques needed to ride with control and speed.

Want to get started with proper riding technique to quickly build your off-road confidence? I want to help you with my free training guide. Click here to download it.