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.
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
- 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.
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…
- Remove stock TTR carb and throttle cable/elbow
- Remove VM24 throttle cap and drill hole up to 5/16″
- Install the stock elbow onto the VM24 cap that you just drilled
- Adjust your needle clip while it’s out, then hook up the throttle cable and scew on the cap
- Install new main and pilot jets if needed
- Remove the old choke cable and one of the throttle cables if you have a newer model TTR
- Fit the VM24 on (making necessary boot/clamp adjustments if needed)
- Route new fuel line onto the carb.
- 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!