TTR125 Mikuni VM24 Carb Swap – Best Bang For Your Buck!

Not too often will you see me devote an entire article on a “Make My Dirt Bike Go Faster!” article, let alone on a bike that is meant for beginners. However, there are numerous advantages to this modification on all Yamaha TTR125 models. If you’ve bought a used TTR125 or have owned one for a while, chances are that you’ve had some problems with the stock carburetor, whether it be a bad choke, sticking float/needle, or jetting problems that make it hard to start or not run right.

You can try different pilot and main jets in there, and even fine to the needle position, but it just never seems to run quite right, and it get worse with time. Some parts of it are just poorly designed and they don’t work right after so many years. Fortunately for those of us that like to fix things, these problems can be cured with a better carburetor. Even better yet, a new one can be found for under 100 bucks!

So, why is the Mikuni VM24 round slide carb better than the stock Mikuni on Yamaha’s TTR 125 four stroke dirt bike? First of all, it’s not as finicky and is easier to jet. Once you get the jetting dialed in, you shouldn’t have to mess with anything other than possibly an air screw adjustment in the cooler riding season. Want more power? Because this carb will give you that, even if your TTR is stock. It will make a bigger difference if you have intake, exhaust, and even engine mods (big bore/cam), but with a stock set-up you’ll get better throttle response everywhere and it will rev out much further, making it feel like a different bike.

Junk Stock Carb
Junk Stock Carb

If you’re tired of messing with the dumb bar-mount choke on the TTR, you can throw that out as well with this new carb conversion. The choke is mounted right on the new carb itself. In fact, a lot of owners of this swap say that it usually doesn’t even need the choke to start, even when the engine is cold.

What Do I Need for This Swap?

  • Mikuni VM24/ss carburetor
  • Screwdriver
  • Pliers (needle nose)
  • Drill with 5/16″ drill bit
  • New jets (Depending on where you get the carb)
  • New 6mm Fuel Line (1′ is plenty)
  • File or new clamp?

How Do I Install It?

Technically, I wouldn’t call this a ‘bolt-on’ swap because there are a couple 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. 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.

Now you will need to drill a larger hole on the new VM24 carb cap so the TTR cable/elbow can fit through it. Some people say they used a 1/4 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 it, 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.

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. 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.

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. 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 that 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.

New, Better Carb
New, Better Carb

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.

Where Do I Find The VM24 Carburetor?

Fortunately, these carburetors are very easy to find and buy. Not only does Mikuni make a lot of them, but you can also find them off of used dirt bikes for cheap. 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, it 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. If you’re lucky, 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. On a side note, I recommend not buying a Chinese knock-off carburetor. They make inferior parts and will more than likely cause problems down the road; just the opposite of what we are trying to do with this swap.

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 Mikuni VM24 from Sudco or from an eBay seller that sells new ones, they come jetted fairly close, although you may need to swap out a jet.

Like 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.

Below are average starting points for the two different VM24 carbs you can put on your TTR125. These are based off of an elevation of about 1000 feet, and a temperature of 70 degrees Fahrenheit. The engine is stock, but it has an aftermarket exhaust.

New VM-24 From Sudco:

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

Used VM24 From KX65:

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

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.

Was that too much reading to remember? I’ll give you a quick low-down on what this swap entails…

  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 newer model TTR
  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!

If you have any questions or comments, or think like something is missing, feel free to to email me or post a comment on the article below. Good luck, and ride safe!

-Tom Stark

34 thoughts on “TTR125 Mikuni VM24 Carb Swap – Best Bang For Your Buck!”

  1. Thanks for this post. I have purchased a Mikuni VM24 from ebay ($60), and fear its a knockoff. It has the “Mikuni” stamped in the casting, but it does not look 100% like other Mikuni VM24s I have seen online.

    Anyway, Im trying to dial in the jetting but none of the Main jets I purchased fit. The threads are just a hair too big to screw into my carb. Also, the Pilot jet for a VM24 is a completely different shape and size than what is in my carb.

    Do you know anything about this? Do you have any ideas on how I can make this carb work since I already have it?

    Thanks- Drew

  2. Tom, I tried to mount the vm24 but ran into 2 problems, one, the gas line intake is at the same height or higher than the peacock valve, second the intake boot will not hold that carb in place, can’t image riding this bike with this carb barely on. I did not use the snorkel as I have a uni filter but even with the snorkel on, which I did not try I can not see this thing staying on.

    1. I’ve installed 3 of these carbs on a TTR125 and never had the height problem you’re speaking of. Did you actually hook it up, turn on the petcock and not see gas flow into the carb? Which side of the carb is the boot not clamping it tight enough? If it’s on the engine side, did you already try removing that small spacer on the clamp so you can turn the screw further?

  3. I purchased a used TTR125 year 2007 on ebay most mikuni vm 24 carbs seem to be made for 200-2004 will they work on mine?

  4. Hi Tom,
    Thanks for your time and support on this mod. I followed your method exactly (Niche Cycle Orig Mikuni, 17.5 and 105 jets) and our 2003 TTR started on the first kick. Awesome. From there things went downhill.
    For some reason the bike just wants to idle high when I restrict the airflow and sputters when I open up the airflow. Ant ideas?
    If I cover the intake side of the carb, or close the air screw (full right), the bike over revs. I tried a larger pilot. No luck there (somewhat expected). It just boggles my mind that the bike starts cold without a choke, but then just loves being starved for air. Anyway… if you’ve got any ideas, I’ll gladly listen. Munk.

    Oh… and I tried smaller and larger mains as well.

    1. Hmm, that is quite the dilemma you have, Munk… Thanks for the comment, and I hope we can figure this out asap! First two questions I have are, what does the spark plug look like, and are there any mods done to the intake, engine, or exhaust? Starting cold without choke naturally would mean that the pilot is too rich. Have you tried a 15 pilot? I have a 17 in my ’03 and it is still rich (haven’t had time to mess with it lately, let alone ride). I originally started with a 25, but I’m also running a kx65 carb. What needle do you have and what is the clip set at? Lastly, how does the bike actually run when riding through the gears? Was the bike fully warmed up? Any bogs or hiccups at certain throttle positions?

  5. Thanks Tom… I think I’m getting closer.
    1) I made a noob mistake and had the idle at full idle rather than minimum at start up… the consensus with the gear heads at work was that I was actually pulling air through the pilot circuit which aspirated enough fuel to rev the engine
    2) After recognizing my first mistake, I was able to get a nice idle with the clip in the most lean position and the airscrew out 2 turns (no high rev) but the engine would bog terribly at 1/4 throttle. After doing further research on Thumper talk, it looks like the newer VM24s from Mikuni (VM24-512) seem to be happier with a larger pilot and main (some contributors are finding luck with 27.5 and 150, respectively.) I have a couple of different sizes arriving tomorrow from Sudco. And I’ll experiment with different needle heights as well.
    Thanks again for a great write up.
    Munk

    1. No problem, Sorry I couldn’t help you sooner. Yes, I believe both of my TTR’s have the needle clip at the leanest position. Mikuni must have made some internal changes to their VM24 in a certain year because one model likes it lean, and the other likes it richer. It sounds like you’re headed in the right direction (my 2000 has 27.5/155 jets and it could probably go down to a 150), and I’m glad you figured out the minor mistake of having the idle too high before throwing a fit… Let me know if you need any more help!

  6. Hey Tom,
    Stuffed the 27.5 and 150 in today. The bike baubles a little at idle as if it can’t seem to find its perfect pitch. BUT wow! What a difference in how the bike runs. My boy is very pleased. Thanks…
    Munk

  7. hey i was wondering if a 17.5 pilot jet and a 160 main jet would be good for a 140cc fourstroke pitbike motor… i have a vm24ss off of my old kx60 and i was hoping that it would work on my motor. thanks for the help

    1. Every bike is different, so I can’t tell you what the optimum set-up is. My guess is that it will be close enough for the bike to run, but you may have to go up or down a couple sizes depending on how it runs. Make sure the jets and passages are clean before installing in case it has gummed up.

  8. I picked up a VM24 round slide from Amazon. I installed it following the instructions above. I couldn’t get rid of hesitation via adjusting the needle clip and experimenting with the air screw (some say it’s actually a fuel screw, but the Mikuni website’s parts diagram shows that it’s an air screw). I get hesitation from idle (pilot jet?) and bogging in the mid-range. The plug looks good so I’m thinking that it’s a jetting problem. I’ve seen on TT that some go to a 17.5 pilot/105 main. This page seems to recommend 27.5 pilot/150 main. Suggestions? Also jetsrus lists two different possible pilot jets (14×4.9mm and 9.5×3.9mm). Which one is it?

    1. What is your current jetting? If There’s a hesitation when you crack the throttle then your pilot jet is probably too lean. This page shows two different starting points for jetting because there are at least two variations of the VM24. I’m guessing the one you bought off of Amazon will require jetting closer to the lean specs.

  9. I had a difficult time figuring out the jet sizes. Using my calipers and a soft light behind the hole, my pilot appears to be a 15 and main is 95. It’s hard to tell for sure and there are no markings on the jets that I can see.

    1. Do you have a magnifying glass? The numbers can be hard to read, but should be visible, especially if you bought it new. If there are no markings/numbers, that would lead me to believe that the jets are knock-off in brand. If 15/95 jet sizes are correct, I would suggest going up at least one on the pilot jet (17.5 is next highest), and possibly bigger on the main jet as well. However, do only one thing at a time so you know exactly what each change is doing.

  10. Tom, I appreciate all your help. No markings on the jets led both of us to believe they are clone jets. I’ve read on-line the clone jets also have a different thread pitch (I haven’t found anyone who could say what the clone thread pitch was). A machinist measured my jets’ thread pitch at 0.80. Mikuni VM24 jets should be 0.90. This confirmed I (unknowingly) bought a clone. I bought it via Amazon, so I’ve requested a refund. Meanwhile, I’ve had a local dealer order me a genuine Mikuni . I’ll let you know how that goes.

  11. I got the VM24-512 from Sudco and changed the jetting to 17.5/105, 3 turns out on air screw, needle 2nd slot from top, 70 degrees, and @ 500′ asl.
    The TTR-125 started and idled nice with no choke. I let it warm up a bit and tried to ride and it choked and quit and wouldn’t start. The plug was black and fouled.
    I dropped the needle to top clip, new spark plug. Started and idled fine but won’t rev. above 1/2 throttle.
    What to do now?

  12. Hello, I installed a VM24 on my wife’s ttr125 and am having a huge problem getting the thing to idleRight now ihave a 110 main and a 17.5 pilot with the needle set in the cleanest position. Any advice on what I can do to get this thing running properly?

  13. The new carb., VM24-512 #001.018 came with
    1.5 slide, 5I14 needle, 864 N-8 needle jet.
    I changed pilot to 17.5 and main to 105
    I’d really appreciate it if someone could check theirs.

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