Having starting or running problems with your TTR125? There’s a good chance that the stock carburetor is your problem. Instead of spending hours trying to fiddle with it, a carb swap can save you a lot of headaches and make your bike start easier and run much cleaner. Here’s a step-by-step video tutorial on how to convert to the Mikuni VM24 carburetor. If you’re unsure about this conversion or have questions on why you should do it, read all the details on swapping out the stock carb HERE.
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!
Time to start putting the big stuff back on to make this a rolling chassis… So far I really don’t have as many hours in this project as I anticipated. Although, it’s far from being over. To see what I finished in my previous article of this build, go to Part 4 Here.
First, we’ll put the rear shock and swing-arm back on…
Then comes the front-end and wheels to make it a rolling chassis with an engine.
Now I want to share with you some of the issues I’ve come across on this conversion build, and try to solve them so everything properly functions. First of all, the brake pedal has some interference with the kick-starter shaft. This is probably the easiest to solve, as you won’t need to make your own pedal like other builders did with the CR85 chassis. The brake pedal still operates, but you can just grind a small notch in it to make clearance for the shaft.
Next problem, which I’m still trying to figure out, is the airbox/intake. I want to make everything look as close to stock as possible, so I’m still using the XR100 intake boot from the carburetor to the cylinder head. The RM85 airbox outlet for the intake boot is on the wrong side of the shock, and I can’t flip the intake boot because there is no clearance as it sits. I may have to modify/chop a section of the airbox out. Right now I have a pod filter on it right now for testing purposes.
And for the part you may or may not have been waiting for…… Since I do not like the look of the out-dated RM80 body-work, I decided to go the extra mile and convert it to the newer and much better looking RM85 styling. I did research on what parts are the same and to see if anyone else has converted theirs. I found out the front fender is about the only bolt-on for the newer plastic, and I couldn’t find anyone else that had done the conversion. That’s too bad… Because you’re going to learn how! If you’re already doing the XR100 conversion, the process to convert to RM85 plastics isn’t any harder than what you’re already doing; just takes a little patience and fitting of parts.
I don’t know how many hours I spent trying to figure out what I need to do, but I think it will be well worth it. This was because the 80 vs. the 85 parts looked the exact same, so I said screw it and ordered a tank, shroud, seat, side panels, and airbox off of eBay After setting the tank and seat on the dirt bike, there were no mounts/holes for the rear of the seat to bolt to… So I took another look at an RM85 frame, and it finally dawned on me. Doh! There’s a post on the frame under the rear of the seat where it and the side panels bolt too! Since I’ve already done a lot of modifications to the frame, I figured this would be fairly simple. On the flip side, I made yet another mistake. The last year of the RM80 was 2001, but it got upgraded side panels, as well as fenders in 2000, although the tank/shroud remained the same. I didn’t know this when I ordered a seat from an ’01 RM80, but it doesn’t fit on the 2002 and newer RM85’s, nor a 1999 and older. This is a mistake that cost me, but now I know that they don’t match.
I used some scrap steel to make this. It’s about 7 gauge material, so it’s strong enough to hold up your rear-end. I just measured the distance between the seat mounts, cut, welded, and kept grinding the post until it fit, and the plastic bar touched the frame (see pic). I’ll drill the holes after I find out exactly where it’s going to be welded onto the frame.
That’s all for today. I just ordered the last of the plastics I need for the conversion, so the next update will be making everything fit together, and possibly the start of a custom exhaust… I’m hoping to have this bike 100% ride-able with just two more updates, so don’t forgot to come back and check soon!
This ultimate pit bike build is slowly but surely coming together… If you read the Previous Article on RM80/XR100 conversion bike, you’ll see that I had just welded up the frame cradle and set the actual XR100 engine in to make sure it fits. There are still some important features I have to add to the frame, but other than that, it’s just a lot of odds and ends to get the bike up and running.
The cradle is done, and the engine can plop right in, but first we need to strengthen it up. Why do we have to take the time to add more structure to it? This is an adults pit bike, and the whole reason we do these conversions is to make it a better performing bike that’s built to last! If it isn’t any stronger than the XR100 chassis it’s just going to break going over jumps, and may result in an injury for the rider.
I added a ‘gusset’ to the front of the frame cradle/down tube. I saw this on most of the other pit bike conversions, so I figured it was important to have. I just used a scrap plate of metal and cut it down to fit over the square tubing. (Don’t worry, I will be cleaning up my welds and the rest of the frame before it’s completely finished). Always remember to clean/grind the metal that you’re going to weld, otherwise it won’t get any penetration and will crack under load.
Next thing is to add/replace the upper frame tube that goes right over the engine. The original one was already cut off when I got the bike, so I just had to bend a piece of tubing and weld it in there. Since I didn’t have a great bender it took a while to get the piece to fit, and even then there’s less clearance than I would like.
This will be a lesson for you, and for my next build. Although I can probably bend it a little more, I will still probably be pulling the engine out to do any work on the top-end. If I really wanted to take the time, it’s not too late to adjust the mounting location for the engine. If I were to do it again, I would lean the engine more forward so it sits within a half inch of the down tube, and that alone might lower it enough to be able to remove the valve cover. Fortunately, the XR100 has such a simple layout that it’s just a few bolts and the engine is out.
Now That the engine cradle and mounts that I machined are all ready to go, it’s time to stick the actual engine back in and start routing electronics. There was just barely enough room in the frame above the cylinder head. Since this is a budget build, and I didn’t want to use the mounts off the XR100 frame, I cut up a couple pieces of scrap sheet metal, bent them, and then ground them until they just barely slipped through the rubber fittings. I put a small ‘hook’ on the ends of them to help keep them in place.
Once I got those bent and fitted, I had to make a spot for the ignition ground to bolt to. I’m as cheap as it gets, so I just took a nut that the ground bolt fit in, and welded it to the inside of a washer.
Now here’s how it looks with all the mounts welded up. Looks like the electronics are ready to go, so we can move on to installing the bigger components…
I know it may not “look” like much, but these are some of the key components of this conversion build that are important to know if you’re considering starting an awesome project like this. Look for the next article soon, as I’ll be putting on the suspension, wheels, and other parts that make it look more like a dirt bike. Not to mention the start of my own little conversion in itself…. I’ve already said too much, so stay tuned.
Part three of this ultimate XR100 pit bike build was probably the most fun for me, even though the bike won’t be finished…. Why? because I got to make my own frame cradle for the engine. You’ll never appreciate something as much as you do when you are able to build it all by yourself. Even though you start out with a complete bike, this project makes it feel like you built your own bike from scratch.
If you haven’t read part 2, read that first HERE!
In order to make a frame cradle for the XR100 engine, you have to cut out the one that’s already there. I started by chopping the down tube off…
There’s a few different options you can choose from, such as round or square tubing, and what size to choose from. Square tubing is the way to go because it’s flat and much easier to make engine mounts for. With that said, feel free to use whatever tubing you like (remember, this is YOUR pit bike build). I used 1.5″ OD (outside-diameter) square tubing because that’s what we had.
I just centered it on what was left from the existing down tube. Make sure your piece is long enough; it’s much easier to cut a chunk off later then to weld an extension. After the down tube is in place, it’s time to mark spots for bolt holes for the engine mounts.
Next I cut what’s left of the lower cradle rails. I am going to bend them towards the down tube like a full-size motocross bike would, so I want to start from scratch.
This is where every build is a little bit different (unless you have machinery to make the exact same bends every time). A tube bender is very handy when you get to this point. However, this is a budget build, and not everyone (including me at the time) has the extra cash to buy a good one). A cheaper, and less than professional-looking way to do it can get the job done, which you may have to do if you don’t want to buy a bender. If you know something with one you could probably get the two pieces bent for pretty cheap though. Like I said before, I wanted mine bent because it looks better and there’s less welding. I didn’t feel like shelling out hundreds of dollars for a quality tube bender at the time, so I opted for the poor man’s bender… Yep, I bought I conduit bender that would fit my size tubing (Hey, it’s budget build, right?). It took a couple of tries, but after taking my time I bent my two pieces without having kinks in them. Before I actually bent and cut the pieces I took a piece of welding wire and put it up to the frame so I could get a model of what the bend should be like. It really doesn’t have to be much more than a 45 degree bend because you can put an angle on the tube where it meets the frame down tube.
Now it’s time to line them up the best I can (they aren’t exactly the same bends) and tack them to complete frame cradle… I just lined the straight part of the tube up with the left over tube on the rm80 chassis.
The Engine cradle is now welded together, ready for the engine mounts to be completed…
Just to make sure before I move on, I tossed the engine back in.
In the next article, I’ll be adding a couple needed things to strengthen up the frame, which I highly recommend if you plan on jumping this bike (who wouldn’t?!). I’ll also be putting the actual XR100 engine in the frame and getting the electronics set up, so stay tuned, because you don’t want to miss any part of this budget pit bike build!!
If you’re looking for Part 4 of this build, Here It Is!