Kx125 low end improvements

I wouldn't go as small as a 12T countershaft sprocket. The angle that the chain comes off the swingarm becomes too sharp and you'll go through chain sliders quickly, and if you don't catch it you'll go through a swingarm. Go bigger on the rear if that's what you're after.

And remember, you're riding a 125. It doesn't have "low-end." They're designed to be ridden way up top. It's just the nature of the beast.

Shane

You can use a 12T if you want to. Some Rm125's came with a 12T factory. don't know about the KX but using the final drive ratio (need to know individual gear ratio & sprocket drive ratio) you can figure out exactly what you're going to get. For example, if you have a 12/49 in 2nd gear it would be roughly equivalent to 14/49 in 1st gear. The 13T is the 1/2 gear in between. If you play with 12, 13, 14 sprockets you will find that the 13 is the best. 12 runs out of gear all over the place, 14 you can't carry enough gear into the corners.

The most important thing with a 125 is your jetting. Most importantly, every throttle position before wide open. You should be able to roll on the power smoothly. There shouldn't be any delays in power delivery, or hesitation when you initially twist the throttle or before you get on the pipe. Make sure your top end is in good condition, power valves working properly, silencer packing fresh. The bike should not smoke excessively at operating temp.

I weigh 185lbs in the buff on a good day. I ride my 125 with stock 13/49 gearing (not for lack of options). Before focusing on my jetting I was clutching out of every corner. After getting my low-mid range jetting spot on I don't have to anymore. I don't use 1st gear anywhere, which includes anything from the mx track to tight wooded trails. I see so many people with 125's that are loaded up on the low-mid, smoke everywhere and spooge dripping down the silencer, no chance of having any power below the pipe.

It would be good to start with the year of the bike and any pertinent mods as they can slightly affect jetting. Also what premix ratio you're running as well as your current jetting specs:
Pilot jet
Air screw
Needle code
Needle clip
Slide cutaway
Main jet

And of course having your float set correctly.

The stock jetting for your bike should be:
(USA)
Pilot jet: 40
Air screw: 2.5 turns out
Needle code: 6CHJ10-82
Needle clip: 3rd (from top)
Slide cutaway: 5.75
Main jet: 420



With that said, most bikes are jetted rich from the factory. It is impossible for the OEM manufacturers to know what altitude, temperature, and terrain that their bikes are going to be ridden in. They typically jet rich for the worst case scenario. Without knowing any of the specifics of the KX engine I would start out by seeing what is in your carb currently and compare it to the stock figures. Also pull the spark plug and look at it's condition. You will likely only need to change your pilot jet and main jet to get the bike running well, as the stock needle and slide are usually sufficient.

For example, let's say that your bike has a 35 pilot and 400 main jet already installed by the previous owner, however you inspect your spark plug and notice that it is black and wet. This tells you that the bike is running rich overall. This does not tell you the engine's response in relation to the main jet.

To test the main jet you want to warm up the bike fully with the spark plug you're currently running. once you have ridden the bike and have the engine up to proper operating temp you want to find a long stretch of dirt road or trail that you can get maximum speed on. Take a 13/16 wrench and a new plug (get a cheap one as it doesn't really make any difference, just make sure the heat range and spec matches a good plug suggested by the OEM) stuffed into your boot. Once you get to the beginning of the straightaway you want to swap out your old plug with the new one. Kick start the bike and go wide open through as many gears as you possibly can. Before you have to start braking hit the kill switch when the throttle is still wide open, clutch in and brake to a stop with the engine off. Remove the plug and re-install your previous plug. The new plug will tell you what your main jet is doing. Too much, or too little fuel.




Once your main jet is properly adjusted you can then adjust your low-mid. Personally I first adjust the idle. The bike should NOT start when cold without the choke. When warm, the bike should idle without any assistance. Once you have the bike starting and idling correctly you can focus on tuning it in for actual riding conditions.Try to find a nice small turn track somewhere, where you can go over the same terrain over and over again. You don't want powdery sand or mud, hard pack is best.I use 2nd gear when doing this because it is responsive enough to show you improvements, but not so short that the difference can't be felt. Again, stick a small screwdriver in your boot and start burning some laps.

Turn the air screw OUT to lean the air/fuel mixture. At the beginning I typically do 1 full turn increments if the bike feels sluggish when rolling on the throttle. Don't get on the pipe, just focus on how the bike feels from no throttle to rolling on the gas. You will become very aware of how responsive the engine is. Once you have turned OUT the air screw enough so that you feel improvement start moving in 1/2 turn increments. You can only go 3 turns out on the air screw safely, so if you hit 3 turns out you need to go back to the pits and swap in a leaner pilot. When you go to a leaner pilot you re-start your air screw setting, usually at 1.5 turns out. The bike will feel more responsive, and more powerful up until a point that you notice a slight decline. STOP there, you have reached the point where you have adjusted the air screw too lean. You want to go back 1 setting to where the bike had the most power and response, and write this setting down along with the altitude and temperature at that time. You can use this information later to help jet the bike when the temperature or altitude changes. At this point you should be able to roll on the throttle smoothly and have a nice clean transition from bottom/mid to on the pipe. The bike should rev freely, and you probably won't notice any smoke from the bike when you're riding (at operating temp).

If you feel odd hesitation somewhere around 1/3 throttle or so, consider dropping the needle in the clip. The pilot jet, air screw, needle, needle clip, and slide cut all work together to give you the low range response that you need.

Getting the bike dialed in takes hard work and a lot of time & patience. In the end the results are well worth it, especially on the 125.