13 Best TTR125 Mods To Go Faster

Modifying a and building up a dirt bike can be just about as much fun as riding it. The TTR125 has many simple mods to get much better performance out of it, especially if you’re an adult or have gained enough riding experience to move past the “beginner” stage.

We’ll look at the 13 best mods you can do to your TTR125/TTR125LE to make it go faster, and be a safer ride to give you more confidence in your riding!

Suspension/Frame Mods

Getting the forks and shocks to behave are extremely critical to get a comfortable and compliant ride if you’re an adult riding a TTR125. The suspension components on these bikes were made with simplicity and cost in mind. In order to make a bike inexpensive to buy off the showroom floor, there’s little budget for R&D and technology advancement.

Fork Mods

The stock fork setup is basically like a pogo-stick if you’re doing more than riding on flat ground in first gear. The springs are soft and the valving is soft and one-dimensional. In other words, there’s no progression of stiffness as it compresses through the stroke. If you hit a log or rock at too high of a speed, it will just blow through the stroke if it’s too soft for your weight.

There’s a few different things you can do. They range from just under 100 bucks to close to 1,000, depending on how you put it together, but let’s start with the cheaper options.

Springs and Oil Change

The stock fork springs are great…. if you weigh 80lbs. Stiffer springs (amazon) are a nice and fairly cheap mod to stiffen it up for a better ride. A fork rebuild isn’t too difficult if you have an OEM manual, proper tools, and some mechanical motivation. Below is a video on how to rebuild your TTR125 forks.

A pair of heavier duty fork springs are an easy way to increase spring rate and stiffen up your suspension, but it’s not always the best way to go for your TTR125. A pair of stiffer springs may actually be too much, causing a harsh ride if you weigh under 160lbs and don’t do jumps with this bike.

Instead, to get a stiffer, yet more plush fork action to help get rid of that pogo-stick ride, you can use one stiffer fork spring and heavier weight fork oil. That’s right, one stiffer spring and one stock spring will still increase the spring rate over stock, and the heavier fork oil will help with the damping.

Cheap/Free Re-Valve?!

A heavier oil will slow the damping/compression of the fork down as you go over objects. It’s kind of like the poor-mans re-valve. You can also tune the oil level (air gap in the forks) to help with bottoming resistance. Just remember to stay within the recommended factory tolerances or else you risk doing damage or blowing seals with too little or too much oil in the fork.

To save even more money, synthetic motor oil can be used as a fork oil substitute. I have personally used it in some of my forks as per recommendation of Bruce Triplett, the old-school suspension guru.

Riding a pair of modified TTR125’s

Shock Mods

Much like the stock fork, a stiffer spring can be used to accommodate a heavier or more aggressive rider. New shock springs (amazon) are around the same price as fork springs.

The next expensive mod would be to swap the stock shock with one from a TTR125LE model if you have a standard or 125L model. The 125LE shock has a remote reservoir and is adjustable. You can rebuild it or send it to a professional to get it re-valved for your weight and riding style.

The last option that’s easy to do (there’s other shock swaps but they involve frame modifications) is to just buy a works shock for your TTR125. They perform much better than stock, but are a lot more expensive.

Don’t Snap Your Frame In Half

A frame cradle is extra support to the frame of your TTR125. If you are a heavier rider that surpasses the OEM weight capacity, you risk bending or cracking your frame. This is especially true if you ride aggressively.

A broken frame while riding could not only result in costly repairs, but there’s a significant risk of injury if the damage is severe enough to cause you to wreck.

I have a BBR frame cradle (Amazon) on both of my TTR125’s. It’s just cheap insurance if you’re going to ride them hard.

Tires

Everyone knows you need tires sooner or later, but this is a more important mod or upgrade than you might think. Having fresh rubber will make a world of difference in handling if your tires are worn or even over a year old!

Soft rubber gives you more traction, especially when the terrain is hard or slippery. Tire rubber gets harder as it ages, and the loss of traction not only makes riding riding more difficult, but also more dangerous.

kids dirt bike
TTR125L

A rear tire will slow you down from a lack of traction, but a bald or hard front tire can easily lose traction and cause the front end to wash out. These are the kind of accidents that have you on your face so fast that you’ll be wondering what in the world happened! Yes, speaking from experience here…

Intake/Jetting

Before doing any mods to make your TTR more powerful, you need to learn the basics of how to tune a carburetor. Jetting a carb may sound intimidating to you if you haven’t done it before, but it’s really not that complicated to learn what some simple jetting changes can do to how your bike runs.

The TTR125 is usually pretty lean from the factory, so any intake or exhaust mods you do will probably make that lean condition worse. Lean jetting will make the bike harder to start, not run as smooth, and often pop/backfire on deceleration.

A TTR125 usually runs lean from the factory. If it’s hard to start, pops or crackles on deceleration, or takes along time to warm up, this is probably because you need to go larger on the pilot jet.

Free Air Box Mod

Once you’ve learned how to re-jet your TTR125, you can now start making modifications that will actually make a difference in power and throttle response, because that’s what we’re going after, right?

The first and easiest mod you can do is removing the air box lid. This can be seen on top of the air box when you remove the seat. The air box keeps dirt out of and getting sucked into your engine through the carb, but if there’s not enough area for air to get in, you’re missing out on possible power gains.

The bigger and more powerful an engine, the more air it needs to suck in. Removing the air box lid greatly increases the area that air can get pulled in.

More Air + More Fuel = More Power

If you want even more air you can drill or cut 1″ diameter holes in your air box and cover the inside of them with foam to help keep dirt out. Just remember that the more holes and area you have open on your air box, the more dirt and dust will enter in. This is why it’s important to have a good, clean air filter that can keep more dirt out of your engine while allowing more air in.

Or there’s always upgrading to a carb that will automatically give you more air and power when properly jetted…

Easy Carb Swap Better In Every Way??

The stock TTR125 carb is lacking, to put it lightly. The choke knob can be nice to have on the handlebars, but it’s not reliable long-term. The stock carb gets dirty internally and the passages gum up easily because they are so small.

A dirty carb also makes it very difficult to start the engine, whether it’s kick-start or electric start. The stock carb is also very lackluster in throttle response and power.

VM24 Carb

Upgrading to a Mikunki VM24 carb is a fairly simple conversion if you like DIY projects. The VM24 carb, when properly jetted, will make your TTR125 start easier, have better throttle response, and provide more power, especially with accomodating exhaust and engine mods.

To view the complete written and video guide on how to swap the stock carb for a Mikuni VM24 and where to find one click here.

Exhaust: Better But Not Loud

Replacing your stock exhaust system or muffler with an aftermarket unit is one of the most common mods on a dirt bike. Why? It is generally very easy and quick to do, but it also makes the most noticeable difference: at least to our ears…

While we often equate a louder exhaust to more power, that’s not always the case. Some exhaust systems are just bigger and simply make more noise, which may sound cool, but they do little to nothing to make more power. What they do well is annoy your neighbor.

A well tuned pipe and muffler will give you more power and still not be obnoxiously loud. The TTR125 can benefit from a better flowing exhaust because the stock exhaust is fairly restrictive.

A good aftermarket pipe and muffler is the Dr. D system (amazon) because it’s larger and flows better, but the sound level is not obnoxious.

Engine

The engine is the life source for making your dirt bike go fast and can make the biggest difference in adding more power, but just because you spend 300 bucks on one part doesn’t mean it will make a noticeable difference. More on this later.

In this section we’ll be looking at all the possible mods you can do to your TTR engine internally that can make a big impact on the power output. However, I just want to remind you that if you don’t tune the carburetor jetting accordingly, you might not see the gains that you were expecting.

Lighten Your Flywheel

Let me first clear the air and tell you that adding or removing flywheel weight does NOT change the horsepower or torque of your engine. Even though you may hear from friends or others on the internet that they gained power by lightening their flywheel, that is actually not the case, technically speaking.

The weight of the flywheel changes the inertia of the engine. For example, if you take two bikes under the exact circumstances while riding at the same speed in the same gear but with different flywheel weights and let off the throttle, the bike with the lighter flywheel will slow down quicker because there’s less weight/inertia to keep it going.

Vice versa for accelerating with different weights. The bike with the lighter flywheel will accelerate faster because there’s less weight to move/rotate.

Pros of Lighter Flywheel

  • Revs quicker
  • Faster acceleration
  • Quicker throttle response
  • Less engine/rotating weight

Pros of Heavier Flywheel

  • More stall-resistant
  • Rolls over obstacles easier
  • Helps prevent wheelspin which equals more traction.

Is A Lighter Flywheel Worth It For My TTR125?

With just about every mod, there’s two sides to its characteristics. For the TTR125, I would recommend a lightened flywheel mod to anyone unless they’re a beginner rider that’s still learning how to ride and use the clutch.

The heavy stock flywheel will help prevent stalling, and if the conditions are slippery then it will also aid in the rear wheel hooking up for more traction.

On the flip side, the stock TTR125 flywheel is so heavy that the benefits would greatly outweigh the negatives for most riders (that may be different on other bikes). The engine has so little power and throttle response that going lighter won’t make it that much harder to ride.

To buy a bolt-on lightened flywheel for your Yamaha TTR125 you can click on the eBay listing below. But before you order one, make sure you know if you need one for an electronic start TTR125LE or kick-start only TTR125 model, because the flywheels are slightly different.

Clutch

Another weak link on the TTR is the stock clutch. If you weigh over 100lbs and ride at moderate to aggressive speeds you will wear out the clutch fibers because they were not built for that kind of abuse.

Signs of Weak/Worn Clutch

When there’s less distance to engagement in the clutch lever, the clutch engages really quickly (as far as lever movement goes), or there’s clutch slippage under heavy acceleration are all signs that your clutch is wearing and should be adjusted or replaced soon.

Free/Cheap Clutch Mod

To fix the weak clutch, there’s a couple of simple options. The first one is to add a washer under each clutch spring, so you will have to remove the case cover and take out the bolts and springs holding the clutch basket in.

Adding washers/shims that are .030″ thick (.75mm) will increase the preload on the springs, therefore making the clutch stiffer to prevent slippage.

It may make the clutch lever slightly harder to pull, but the trade off is worth it if you plan to put a lot of hours on your TTR125.

Clutch Spring Upgrade

If you’re doing clutch maintenance and have it removed, check the spring length to see if they are worn past OEM spec according to the manufacturers service manual. If they’re shorter/worn beyond spec, that is one of the reasons why your clutch may be wearing out quicker.

You can either replace them with some new OEM springs and washers, or you can swap them out for some stiffer clutch springs. They will make the clutch pull even stiffer yet, but I highly recommend this mod if you are over 150lbs and are aggressively riding your TTR125.

Big Bore Kit

Now we’re getting to the meat of the engine with piston and cylinder mods to extract more power. Increasing bore size with a bigger piston alone will give you more bottom end and mid-range power. More on this later.

If you know a local machinist or engine shop that does good work and can hold proper tolerances, the best way to go would be buying a quality big bore piston (to bump up to nearly 150cc) and bring the piston and your cylinder to them to bore it to the correct size.

Cheap Kits Are Just That

There’s a selection of options now that you can choose from to get a bigger bore for your TTR125, some are worse than others.

A $120 kit complete with cylinder and piston parts may work, but the tolerances and quality of material and work just doesn’t compare to USA/Japan/Euro made components. The performance will be lower, and the reliability will be less.

A quality piston will be $100-150, or big bore kit will be around $300 for a TTR125. The BBR kit is the standard and has been around for a while. It has everything you need to rebuild the top-end, including a new camshaft, which we’ll cover next.

BBR Kit – Don’t buy it without knowing this!

If you decide to go with the BBR kit to get more power, make sure you check the threads for the timing chain tensioner. They have been known to have quality issues that cause the bolts to strip out. If you happen to get one with bad threads, contact the seller, or re-drill and tap the holes for the next size larger bolt.

Other than that, the BBR kit will have all of the proper tolerances to give your bike a well-performing and long-lasting engine.

Camshaft

The camshaft can make the biggest impact on what the power-curve of your engine runs like. The stock cam, on most air-cooled 4-strokes, is very conservative. Low lift and low duration that produce smooth and low numbers.

You’ll never have clearance issues of the internal engine parts, and running low octane fuel will work all day long.

Where Do You Want The Power?

When swapping cams, you can buy one that shifts the power for more mid to upper RPM power, or more low to mid for more grunt. It depends on the selection of aftermarket availability and what kind of riding you do.

Do You Know The Specs?

I always like to know all the numbers of the new camshaft I’m looking to buy. That way I can compare it to a stock cam and know exactly what it will do.

To put it in simple terms, if I want it to be a single-track woods bike, I look for a cam with more lift, but not much more duration.

A cam with high lift and long duration will be good for a high RPM race engine, but may need some piston valve relief clearancing, and will need supporting mods, which I’ll cover here shortly.

Head Porting – Not All Porting Is Created Equal

Cylinder head porting is often an after thought when consider modifications to make an engine more powerful. Probably because many riders don’t have the time or motivation to send it out to someone. They would rather spend $300 on an exhaust then on porting. It makes the biggest noticeable difference (sound) and is easy to bolt on.

If You Are Serious About More Power

If you actually want to get the most out of your TTR125, or any 4-stroke for that matter, you will come to find out that porting done right is the missing link to making a great running engine.

Stock heads are meant to flow enough air for the stock carb, stock cam, valves, piston, exhaust… So if you go bigger with those parts, the intake and exhaust ports are just going to choke it down and limit the gains.

Think of it as trying to suck a glass of water up with a regular sized straw. Now try doing it with a mini sized straw. It’s much harder to draw out the liquid/air when the hole size is choked down.

Just Bigger Isn’t Better

If you take your cylinder head to the first engine performance shop you find, chances are they won’t know what your head specifically needs to get the most out of it. I strongly recommend you find a shop owner, whether it’s 30 miles or 3000 miles away, that specializes in the engine you have. They will know what it will take to meet your specific build needs.

It All Works Together For The Greater Good

You don’t have to spend a fortune to get a better running and more powerful engine. But, to get the most out of your money, you need to plan out what you want it to do differently than the stock engine.

Are you looking for a low-end torque-based engine that has smooth power for trail riding, or do you want an all out race engine that is “set to kill” for max horsepower?

If you want a torque based engine, mixing a large carb with a max lift and duration camshaft would be counterproductive because they are built to add more power at high RPM. This is why you want to put together a complete package with each component that is designed for the type of power and that you are looking to achieve.

Torque-Based or High RPM Parts List

Below is a standard list of what you should do to your TTR125 (or most any 4-stroke dirt bike) for two different types of engines. It is merely a guideline if you want to get the most out of your money to get great results.

Torque-Based Trail Bike Engine

  • Stock sized carburetor (or slightly bigger if adding displacement).
  • High velocity porting to match the carb and engine size.
  • High compression piston
  • Big Bore and/or stroker crank
  • Exhaust system tuned for torque (small diameter outlet, stepped, long length head pipe)

High RPM Race Engine

  • Big carburetor
  • Porting for high RPM
  • Oversize intake valve
  • Big Bore/high compression piston
  • Exhaust tuned for high RPM (Large diameter, short length)

All The Best Mods

Last, but not least, is the full list of modifications you can do to your Yamaha TTR125.

  1. Fork spring/valving upgrade
  2. Shock/spring upgrade
  3. Frame cradle
  4. Jetting
  5. Air box lid
  6. Air box holes
  7. Carb swap
  8. Exhaust upgrade
  9. Lightened flywheel
  10. Clutch mod/upgrade
  11. High compression/big bore piston
  12. Camshaft upgrade
  13. Cylinder head porting

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4 Responses

  1. Kenny O TEAM CHINOOK says:

    Lots of good tips here. For the flywheel lightning……..is the goal to take off about 1 pound ? Can anyone describe where the cutting on the lathe should be ? Is there concern for balancing ?

    • mxhideout says:

      Thanks for the comment! 1 pound would definitely be a good number to hit for weight removal. Anything over 10 ounces should make a noticeable difference. I don’t personally have any pictures of a lightened flywheel, but the seller on eBay has a picture of one to get an idea. Material is usually removed from the ends of the outer circumference.

      I personally wouldn’t mess with re-balancing since it’s so heavy to begin with. It might have slightly more vibration at idle, but I doubt you’ll notice a difference while riding unless you take off more than 1lb.

  2. Cheyne says:

    This may be a silly question, but with proper jetting would it be possible to run the stock exhaust with the BBR 150 kit? I understand I won’t be receiving the full benefits from the larger displacement, but I would really like to keep this bike as quiet as I can. My friend has a 143cc kit on his KLX110 with the stock exhaust and it runs fine.

    • mxhideout says:

      Absolutely! Yes, you’re right about it not reaching its full potential with the stock exhaust, but it should still be able to run well after tuning the carb jetting.
      You’ll probably gain more low-end and mid-range power with the big bore only, so the top-end may feel like it’s flat and lacking, but it will definitely be quieter. Thanks for the comment!

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