TTR125 Mikuni VM24 Carb Swap: Is It Worth It?

Yes—the genuine Mikuni VM24-512 is one of my favorite TTR125 performance upgrades, especially if you're tired of fighting the stock carb.

I've personally installed and tuned this carb on my TTR125. Compared with the stock carb, I got easier starting, better throttle response, more usable power and more over-rev—and the VM24 is easier to tune once you understand how its circuits work.

You don't need a modified engine to benefit from it. A stock TTR125 can run better with a properly jetted VM24, while bikes with intake, exhaust or engine modifications have even more to gain from the larger carb.

For a stock or lightly modified TTR125, I'd buy the genuine Mikuni VM24-512 rather than a cheap VM24-style clone.

Below I'll show you exactly what you need, how I installed mine, the jetting I used and how to tune it if your bike doesn't run perfectly with the same settings.

	Stock TTR125 Carb	Genuine Mikuni VM24
Starting	Can become finicky	Easier when properly tuned
Throttle response	Mild	Quicker
Power potential	Limited	Better
Over-rev	Limited	Better
Tuning	Small passages / can be frustrating	Straightforward once understood
Choke	Handlebar-mounted	On carb
Installation	Factory	Minor modifications required
Best for	Stock bike that already runs well	Stock through moderately modified TTR125

My recommendation: If your stock carb already starts easily and runs cleanly, you don't need to replace it. But if you're fighting the stock carb or want better engine performance, the genuine VM24 is one of the first engine-related upgrades I'd consider.

This is the genuine Mikuni VM24-512 I used for this conversion.

There are much cheaper VM24-style carbs available, but I'd spend the extra money on the genuine Mikuni if your goal is better reliability and predictable tuning.

Check the Genuine Mikuni VM24-512 price on Amazon →

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Why I Swapped the Stock TTR125 Carb

The stock TTR125 carb can work perfectly fine when it's clean and properly jetted. The problem is that these bikes come fairly lean, and as they age, the small passages, choke, float/needle and other components can become frustrating to deal with.

You can rejet the stock carb—and that's what I'd do if you're happy with its performance.

But I wanted better throttle response and more power potential, not simply to make the stock carb run correctly. That's where the VM24 makes more sense.

The larger round-slide carb allows the engine to breathe better and gives you more tuning flexibility, particularly if you later add intake, exhaust or engine modifications.

Mikuni VM24 Carb vs. Stock Carb

The stock TTR125 carb isn't necessarily bad. Once it's clean and properly jetted, it can work well on a stock engine. But the genuine Mikuni VM24 has some advantages if you want better performance or you're tired of dealing with the stock carb.

I've run this conversion on my own TTR125, and the biggest differences I noticed were better throttle response, more usable power throughout the RPM range, and considerably more over-rev.

Even on a stock engine and exhaust, the VM24 can make the bike feel noticeably different. The benefit becomes even greater if you've opened up the intake or added an exhaust, big-bore kit, cam or other engine modifications.

The VM24 is also relatively straightforward to jet. Once it's dialed in, you generally shouldn't need to make major changes unless your riding conditions or engine setup change. You may only need an occasional air-screw adjustment as temperatures change.

What About the Stock Choke?

One other benefit of the conversion is getting rid of the TTR125's bar-mounted choke setup. The VM24 has the choke directly on the carburetor.

On my TTR125s—and based on feedback I've received from other owners who've done this conversion—the properly tuned VM24 also tends to start very easily. Mine often doesn't need much choke even when the engine is cold.

Not sure if the carb should be your first TTR125 upgrade?
I've modified multiple TTR125s and ranked the upgrades I'd do first depending on whether you want better suspension, power, handling or adult ergonomics.

Next: Best TTR125 Mods That Are Actually Worth It →

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What You Need for the TTR125 VM24 Swap

To replace the stock TTR 125 carb with the Mikuni VM series carb, you need these simple tools to make it easy:

Required:

• Genuine Mikuni VM24-512 carb (Amazon)
• Screwdrivers/basic hand tools
• Pliers
• Drill + appropriate drill bit
• Correct jets
• Approximately 1 foot of 6mm/¼-inch fuel line (Amazon)

May be needed:

• Replacement intake boot (Amazon) if yours is hardened, cracked or won't reach properly
• Different clamp

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How Do I Install The VM24 On A TTR125?

Technically, I wouldn't call this a 'bolt-on' swap because there are a couple of modifications you have to make.

However, this is one of the easiest conversion projects you will find when swapping dirt bike parts. Total time should be 1-2 hours if you have everything ready, and even less if you've done it before.

The first thing to do is take off the throttle cap from your new VM24 carb, as well as the one from your stock TTR125 carb, and remove the cable/adjustment screw so all you have left are the bare caps.

You will be using the stock TTR throttle cable, which will require the metal elbow off of the stock carb cap. It's held in place with a locking clip, so just pull that out with some pliers.

Drilling The VM24 Carb Cap

Now you will need to drill a larger hole in the new VM24 carb cap so the TTR cable/elbow can fit through it. Some people say they used a ¼ drill, but the TTR elbow fitting measured .285", which is about 9/32".

That drill might work; otherwise, you can go up to a 5/16" drill (.312") to make it fit. If you are wondering about the threads, yes, they will be gone after you drill through them, but you don't need the adjustment screw from that cap for it anymore.

Now you can put the elbow assembly on the new cap and hook the TTR cable up to the slide.

What to do if the carb boot is too short

Some owners of this conversion mentioned that the new carburetor is shorter in length, requiring you to stretch the inlet-side boot to make it reach. I did not have this problem when I swapped it onto my 2000 TTR125L; however, you can buy a new aftermarket intake boot (Amazon) that will stretch and fit a little better, especially if your stock boot is hard and cracking, which can cause an air leak.

The clamp on the inlet fits without modification, but the engine side of the carb boot didn't clamp down far enough on mine. You can either get another clamp or just file down the spacer in the stock clamp so you can tighten it down more.

Routing New Gas Line

After you have the carb bolted in, the only thing left is the gas line. You will more than likely need a longer one to reach the new carb.
A ¼" or 6mm fuel line (Amazon) should work fine, and you only need one foot at the most.

You can either wrap it around the back of the frame to keep it out of the way, otherwise you can just route it underneath the frame, which is a shorter distance.

If you haven't already, you can completely remove the stock TTR125 choke and cable because it isn't needed anymore.

Also, for those of you who have the newer model TTR125's with the two-cable throttle set-up, just use one of those cables with the new VM24 carb and remove or tie up the other one, as it doesn't use two cables.

Just turn the gas on now and fire it up! Adjust the idle screw knob when it's warmed up, and it purrs like a mountain lion.

What Jets Should I Use?

Depending on what mods have been done to your bike and where you live, your results may slightly vary. If you're buying a new Genuine Mikuni VM24 from Amazon, Sudco or from an eBay seller that sells new ones, they might come jetted fairly close, although you may need to swap out the jets (jetting specs listed later in this article).

As I mentioned above, if you're using a VM24 carb from a 65cc 2-stroke then it will require different jets to run properly. The 65's need much richer jetting compared to the small-bore four-strokes, so you'll need to change the main jet for sure, and possibly a pilot, depending on what comes with it.

Don't worry, jets are only a few bucks, and they share the same jets as most other Mikuni VM and TM carbs.

With that said, if the float height is not adjusted properly to factory spec, your jetting will be thrown off and will result in some head-scratching confusion.

The float height directly affects all other jet circuits. This will be discussed more in-depth in a later post.

Important: These jet sizes are starting points—not guaranteed settings for every TTR125.

Your required jetting can change with elevation, temperature, fuel, intake/exhaust modifications, engine modifications, float height and even which version of the VM24 you're using.

That's why I'd rather teach you how to recognize rich vs. lean symptoms than tell you to blindly copy my jets.

New VM24 Jetting For TTR125:

• Main Jet: 105
• Pilot Jet: 17.5
• Needle: 1st or 2nd clip position from top

These are based on an elevation of about 1000 feet, and a temperature of 70 degrees Fahrenheit. The engine is stock, but it has an aftermarket exhaust.

Used VM24 Jetting From KX65 For TTR125:

• Main Jet: 155
• Pilot Jet: 27.5
• Needle: 1st/top clip position
• Air screw: 1-1.5 turns out

Don't Just Copy My Jet Sizes

Your TTR125 may need different settings than mine. The fastest way to get the carb dialed in is to understand what a rich or lean pilot circuit actually feels like.

One important difference: the VM24 uses an air screw, even though your TTR125 is a 4-stroke.

That means its adjustment direction is opposite a typical 4-stroke fuel screw:

VM24 air screw: OUT = leaner / IN = richer
Typical fuel screw: OUT = richer / IN = leaner

My free 4-Stroke Fuel Screw Tuning Guide will help you understand rich/lean symptoms and how the pilot circuit responds. Just remember that when you apply it to the VM24, the air-screw adjustment direction is reversed.

Get the Free 4-Stroke Fuel Screw Tuning Guide →

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Troubleshooting the needle jet circuit

The needle is about the only thing that may be harder to tune. It runs a little rich, especially with a bone stock TTR, even at the leanest position.

I haven't found any leaner needles you can buy for it, but after riding the bike for a little bit and allowing it to fully warm up I didn't even notice a hesitation.

Other than that, this bike runs great now from bottom to top with more over-rev due to the larger bore size.

Plug the vacuum line if it's still not running well!

If you followed the jetting specs I listed and it's still not running well, then there may be a vacuum line that needs to be plugged. It's usually towards the front of the carb. It may be sucking in air, throwing the air/fuel mixture off.

TTR125 VM24 Troubleshooting

Symptom	What I'd check first
Hanging/high idle	Lean pilot circuit, air leak, throttle cable/slide not fully closing
Only runs with choke	Pilot circuit too lean or restricted
Black/fouled plug	Rich condition; identify which circuit
Bog at ¼–½ throttle	Needle/needle jet circuit
Bog at wide-open throttle	Main jet first
Runs rich even with smaller jets	Float height, needle/needle jet, verify genuine carb/jets
Idle screw doesn't lower RPM	Slide/cable, air leak, pilot mixture
Jetting changes don't make sense	Check float height and verify jets/carb aren't clone parts

Don't change three things at once. Identify which throttle position/circuit is causing the problem, make one change, and test again.

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Want to Learn How to Diagnose the Carb Instead of Guessing?

Copying someone else's jetting can get you close, but it doesn't teach you what to change when your bike behaves differently.

In 4-Stroke Jetting Made Easy, I show you how to identify which carb circuit is causing the problem, recognize rich vs. lean symptoms, and make the right adjustment without randomly swapping jets.

Learn More About 4-Stroke Jetting Made Easy →

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Bottom Line

1. Remove stock TTR carb and throttle cable/elbow
2. Remove VM24 throttle cap and drill hole up to 5/16"
3. Install the stock elbow onto the VM24 cap that you just drilled
4. Adjust your needle clip while it's out, then hook up the throttle cable and scew on the cap
5. Install new main and pilot jets if needed
6. Remove the old choke cable and one of the throttle cables if you have a 2008+ model (they come with two throttle cables)
7. Fit the VM24 on (making necessary boot/clamp adjustments if needed)
8. Route new fuel line onto the carb.
9. Turn the gas on and fire it up!

Where Do I Find The VM24 Carburetor?

Are you ready to swap out your poor-performing stock carb for a better starting & running Mikuni VM24 carb for your TTR125? Check the price on Amazon here. If it's fouling plugs or only runs on choke, use the jetting specs listed earlier in this article as a starting point.

Tips for buying used if you have more time than money

Sudco and eBay are common places to buy them new. You can also get a VM24 off of a 65cc 2-stroke motocross bike, such as a KX65. They require different jetting, but once they are dialed in, they will run just as well.

They pop up on eBay all the time, and can be had for as little as 30 bucks or less for a used one. You might be able to get away with buying a new jet or two and giving it a good cleaning.

If it has a lot of hours, though, it may require a rebuild kit. This isn't so bad, but it will end up costing almost as much as a new carb.

Genuine Mikuni vs Cheap VM24-Style Carb

You can buy VM24-style carburetors for substantially less than a genuine Mikuni, and some of them do run fine.

The problem is consistency.

Jet sizing, threads, internal machining, float settings and other components aren't necessarily identical to genuine Mikuni parts. That becomes especially frustrating when you're trying to diagnose jetting because the number stamped on a clone jet may not behave like the equivalent genuine Mikuni jet.

If you're experimenting on a very tight budget, a cheap carb may be worth the gamble.

If your goal is to install it once, tune it and ride, I'd buy the genuine Mikuni VM24-512.

• My choice → Genuine Mikuni VM24-512 (Amazon)
• Budget alternative → Cheap VM24-style carb (Amazon)

VM24 or VM26 for a TTR125?

For a stock or lightly modified TTR125, I'd choose the VM24.

Bigger isn't automatically better with carburetors. A carb that's too large can reduce air velocity at lower RPM and hurt throttle response—the exact opposite of what most trail riders want.

The VM26 makes more sense on a serious engine build with a big-bore kit and supporting cam/head, intake and exhaust modifications where the engine can actually use the additional airflow.

So my general recommendation is:

Stock / exhaust / intake → VM24
Moderate modifications → VM24
Big-bore performance build → consider VM26

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Is the Mikuni VM24 Worth It on a TTR125?

Yes. For a stock or moderately modified TTR125, the genuine Mikuni VM24-512 is one of my favorite engine-performance upgrades.

If your stock carb is already properly jetted and you're happy with how the bike runs, you don't need to replace it. But if you want better throttle response, more power potential and a carb that's easy to tune, I'd choose the genuine VM24.

Just remember that my jetting specs are a starting point. Your bike may need different settings depending on temperature, elevation and modifications.

Ready to do the swap?
Get the Genuine Mikuni VM24-512 I Recommend →

If you've already installed it and need help tuning it:
Get My Free 4-Stroke Fuel Screw Tuning Guide →