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Yamaha TT-R125x Visual Jetting and Airbox Modifications Guide


WARNING! All of the information contained in this document is intended for private, non-commercial use only and is to be used at your own risk. If you are not mechanically proficient, or proficient in the use of any of the required tools, equipment or supplies, seek qualified help before proceeding. The reader is responsible for any and all damages, injuries, charges, fees, fines, or other liabilities incurred from any use of this information.


This document was created from the excellent tutorial by ovrrdrive in the following thread:


[Side note: You could probably do this without removing the carb from the bike. However, I wanted the experience of doing it and thought the pictures would come out better, too, so I did.]


1. First, go to the left side of the bike and turn the fuel petcock to the “Off” (horizontal) position. Remove the fuel line, vent line and overflow line from the carburetor. You may need to gently pry them with a screwdriver to break them loose.

2. Next, remove the carburetor from the bike; this is easy to do. Go to the right side of the bike and loosen the screws in the clamps holding the intake boot and manifold to the carburetor. Just loosen them a good bit and the boots will pop right off. Remove the carburetor from the boot leading to the air box, first, and then just pop the carburetor out of the manifold boot. The red arrows indicate the two screws for the boot clamps.

3. The choke cable is the shiny, brass one on the top of the carburetor; it loosens with a 14mm open-end wrench. Loosen it up until it pops up, then gently pull it out of the carburetor. Now, loosen the throttle cable / slide valve / jet needle assembly. I loosened mine with a pair of pliers, then spun it off by hand. After it’s loose, just carefully work it out of the carburetor; it should slide out easily. Be very careful not to nick, scratch or bend the jet needle.

4. After you have the cables removed from the carburetor, it comes right off the bike. Here’s a picture of the throttle cable assembly and the choke cable hanging loose after I took the carburetor out.

5. Now, it’s time for a few gratuitous pictures of the carburetor detailing some of its features…

6. Now, to remove the float bowl from the carburetor… Before you try to loosen the four retaining screws on the bowl, hold a well-fitting screwdriver in the screw head and tap the end of the screwdriver a few times with a hammer to break them loose and make them easier to remove without stripping the heads. I forgot to do that to the drain screw and almost rounded it off. After you remove the four screws, the float bowl may still be held in place by the gasket (at least mine was). The last thing you want to do is pull on it really hard on the bowl to break it loose. If you do, you’ll probably rip the floats out or bend them when the bowl does break loose. I took a small, regular screwdriver and gently pried the bowl in three or four places until it started to come off. You can also tap on the side of the bowl with the handle of the screwdriver to pop it loose. When it does start coming off, be careful not to rip the bowl gasket so that you can reuse it.

Here’s a picture of the internals of the carburetor after the bowl was removed:

…and another showing the float needle and seat below the float assembly:

7. The jets come right out pretty easily. When I started to loosen the main jet, the piece underneath it started to turn, too, so I held it with an 8mm open-end wrench while I backed the main jet out. If there is any dirt, sediment, varnish, scale, or oxidation in the carburetor, clean it out thoroughly. This is one part of your bike that should always be spotless. If you use Sta-bil or Seafoam in every tank of gas (and in every gas can you fill), you probably won’t have to ever do this again. Here’s a picture of the stock #105 main jet on the left and the new #107.5 main jet on the right:

8. Be sure to use a small, well-fitting screwdriver when you remove the Pilot Jet. Here is the stock #15 pilot jet on the bottom and the new #17.5 pilot jet on the top:

9. Screw both the main jet and the pilot jet in snuggly, but do not over-tighten them. I then reset the pilot (fuel) screw at 1.5 turns out (CCW) from the fully closed (CW) position as a starting point. Reinstall the float needle, floats and the float bowl. This would also be a good time to replace the four float bowl screws with stainless steel allen-head screws from Home Depot, Lowes, Fastenal, your local motorcycle shop, or your local hardware store so you never have to worry about stripping them in the future.


10. Screw the float bowl back on and you’re ready for reassembly. The throttle slide has two different slots in it, so you need to pay special attention when you place it back into the carburetor. The wide slot with an angled bottom should line up with the idle screw on the left side of the carburetor.

The angled portion rests on the idle screw. It’s easier to reassemble if you hold the carburetor perfectly level so that the jet needle will hang straight down and line up with the hole in the needle jet in the bottom of the carburetor’s throat. You may need to pull the slide back out a few times before the jet needle drops in correctly. DO NOT FORCE IT!

11. After you have finished reinstalling the throttle cable, insert the front of the carburetor into the rubber manifold on the cylinder. Be sure the clamp is loose enough to allow the manifold to expand slightly and be sure to line up the protrusion on the top of the carburetor with the notch in the manifold. It takes a firm push to seat the carburetor into the manifold. Then, install the rear air boot onto the carburetor. Next, reinstall the choke cable. It takes a fair amount of downward pressure on the nut to get engaged onto the threads. Snug it up with a 14mm open-end wrench, then reinstall the overflow line, the vent line and the fuel line and tighten both of the carburetor boot clamps.

That’s it! Make sure everything is reconnected correctly and everything is tight, then turn the fuel valve back on and give it a go! It really isn’t too bad of a job if you just take your time and do it right. Now, while I had everything else apart, I pulled the spark plug out to see the color of the insulator. It looked like the jetting was dead-on before. I figured if I wanted any improvement, at all, I’d better do some air box modifications to compliment my new jetting. Otherwise, I would be adding a lot more fuel but no additional air which would make it run too rich. I wanted to do the air box modifications, anyway, so here it goes… Here’s a picture of a good-looking spark plug from a properly-jetted engine. Note that the small, round, ceramic insulator surrounding the small, metal tip has a sort of “cardboard” color to it. If it’s white or pitted, then the engine is running too lean (not enough fuel), if it’s very dark brown or black, then the engine is running too rich (too much fuel).

Here is a picture of the top of the stock air box with the seat removed:

12. Wiggle the rubber air snorkel while pulling up to remove it from the top of the air box.

Here’s a top view of the air box with the snorkel removed:

13. On the left side of the bike, remove the three screws holding the triangle-shaped cover on the front of the air box, remove the cover, then pull the screen out. Reinstall the cover and the three screws.

Here’s a picture of the air box from the right side with the screen removed:

After all was said and done, the only parts I had left over were the ones that I was supposed to!

And that, as they say, is that. After I cranked it up, I ended up going out about a ½ turn more (CCW) on the pilot (fuel) screw as it was popping a bit and the change improved things. I’ll need a good riding day to be sure, but I think the #107.5 main jet may not be big enough. I’ll probably change it to the #110 main jet I picked up.


Ride reports initially are that it has more bottom-end grunt and smoother acceleration through the power band. It just feels more like it should; not like it’s being held back.


Overall, these were very easy modifications and worthwhile, I think.


Well, how’d I do? Suggestions, comments?

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