How to put a conical air filter on a Honda CBR125R

CBR125R modified air filter

In this post I am going to show you how to install a universal conical air filter on your Honda CBR 125 R motorcycle. First of all, let's talk about what will change in case you will install a different air filter.

  • Performance: whether the performances will increase or decrease depends on how you will tune the injection timings after changing the air filter. With new PGM-FI (Fuel Injection) motorcycles, the quantity of gasoline which is injected in the cylinder, though the throttle body, depends on how much time the injector is commanded "open" by the ECM (Engine Control Module). In other words, the quantity of injected fuel depends on how much time (hundreds of microseconds, or milliseconds) the injector is open. The quantity of gasoline which must be injected is not random, it depends on the quantity of air (i.e. oxygen) which the piston "sucks" into the cylinder: more air flows in the cylinder, more gasoline you have to inject; this is chemically decided by the stoichiometry ratio between air and fuel, which should theoretically always be around 14 (in terms of mass, 1 part should be fuel, 14 parts should be air). If you decide to install a universal conical air filter, the quantity of air which will flow into the cylinder will change: especially at high engine rotation speed (rpm), since the new "open" filter has a smaller resistance to the air flow, the quantity of air which will flow into the cylinder is be higher, so you will need also to increase the injection timing, otherwise the air-fuel mixture which will flow into the cylinder will be "lean" (too much air compared to the fuel injected), the combustion will be bad, and the engine will have less power. The conclusion is that a modified air filter can give you some more power (few percents, probably), but you need to adjust the air-fuel ratio by increasing the injection time, and this requires adding an additional ECU module (Power Commander, or a DIY ECU as I did). If you decide to skip this modification (no additional ECU module), your performance might not have any positive change, or might even get worse. According to my experience, adding an additional module is not needed if you are performing only this simple modification, because the original ECU is capable of regulating itself according to slight changes in the system (using the O2 oxygen lambda sensor). But if you are planning to modify also other parts, such as installing a 166cc bore up kit, you will absolutely need to install also an additional ECU module to adjust the injection timings.
  • Space/Weight: a modified air filter is installed instead of the original air-box, this means that after removing the original the air-box you will have much more space under the fuel tank. Also, the weight will decrease (-1 kg or so).
  • Sound: after installing a universal conical air filter, you engine will be a bit louder, especially when you completely open the throttle, and at high engine rotation speed (over 5,000 rpm).

Now, let's talk about what you will need in order to make this modification. First of all, you will need 2 filters.

  • Air filter for the throttle body. This is the biggest filter (pink color in the pictures). For this pictures, I used a Daytona universal air filter with a diameter of 45mm (デイトナ(DAYTONA) エアクリーナー パワーフィルター φ45 ラウンドオーバル ストレート 63359). I bought this product for 2,078 Yen (15 Euro) on Amazon Japan. The diameter of this filter is not written in the Honda CBR125R Service Manual, so I had to find it out by myself by measuring the diameter of the original air-box.
    Daytona Air Filter CBR125R
    Daytona Air Filter CBR125R

    Honda CBR125R airbox diameter
    Honda CBR125R airbox diameter

Air filter for the crank case breather. In the original system (original airbox), the crankcase breather is connected to the original airbox, in order to permit recirculating the crank case gases (exhaust gases which go through the rings of the piston during the explosions inside the combustion chamber); this is done in order to be according to the exhaust gases regulations (pollution, ...). If you decide to remove the original airbox and install your own filter, you need to connect something to the crankcase hole (otherwise dust will go in, and dirty gases and oil from the crankcase will directly flow out on your fairings, pedals, frame and so on). For this purpose, I bought a Kijima air filter with 12mm diameter (キジマ(Kijima) ブリーザーホースフィルター ホース内径12mm用 106-552) at Amazon Japan for 1,304 Yen (10 Euro).

Kijima air filter CBR125R
Kijima air filter CBR125R


Kijima crankcase breather filter
Kijima crankcase breather filter
  • Honda CBR125 Service Manual. This manual is very useful also for other maintenance and modifications described in this blog. You can download it here. For additional service info, I recommend you also to buy the Haynes Service Manual for the Honda CBR125R (newest models starting from 2011).

Of course, you will also need all necessary tools to perform this air filter modification. You can follow the manual, which explains you how to remove the fairings, and move up the fuel tank (be careful, the tank must be not full otherwise gasoline will spill out). Once done the previous steps, you will have to remove the original air box and hoses, and install the new filters.

When I performed this modification, the motorcycle was working perfectly even if I did not install any additional ECU (such as Power Commander and so on...). The engine was running smoothly, but I did not notice any visible performance increment. In other words, the performances did not visibly increase and not even decrease. But I remember that the sound of the engine changed dramatically: with throttle closed (example: engine idling) there was no difference in sound between original air box and modified filter. But opening the gas (especially with throttle wide open), the sound became very loud compared to the original air-box. So, in conclusion, I recommend you to do this modification if you want to give a more "racing" and "aggressive" sound to your CBR125R; but do not expect to have 10% more power just by changing the air filter of your motorcycle. If you want to obtain higher performance increases, you should also change your exhaust pipe with a racing one (for example Akrapovic) and install a 166cc bore-up kit (Athena, Malossi, ...).

Author: Davide Cavaliere

I am an Italian Electrical Engineer graduated at Politecnico di Milano. My interests are motorcycles and cars, electronics, programming, Internet of Things, and Japanese culture.

2 thoughts on “How to put a conical air filter on a Honda CBR125R”

  1. Hi I have a Cbr125r 2007 thank you so much for your help for I'm a learner rider I have changed my oil and about to do the filters like this post but waiting on crank case filter now will I need to do anything to the factory ecu?

  2. Hi I have a Cbr125r 2007 thank you so much for your help for I'm a learner rider I have changed my oil and about to do the filters like this post but waiting on crank case filter now will I need to do anything to the factory ecu? Will the 166 bore up kit fit the 125r 2007 model and also would it be a straight forward job thank you so much.

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