In-line fuel filter…

Due to the fact that the CBF125 has no form of visible filtering for petrol, either in the tank (the only part you can take out of the tank is the fuel level float/sensor) or rest of the system (fuel pump is a sealed, replaceable unit and cannot be disassembled), I, and many others have felt it best to add an in-line filter to the fuel hose running from the tank to the fuel pump.

Here’s my example (as always, click the image for a larger version):

If you want to add one yourself, the following information may be helpful:

  • SAFETY FIRST! Petrol, in both vapour and liquid forms is extremely flammable and has a low flash point, hence why a spark can ignigte it! Keep any sources of ignition well away. Petrol vapours aren’t good for your respiratory system either, so make sure you’re working in a well ventilated space. They’re also not good for your skin, so use a barrier cream or appropriate gloves. Also use eye protection because the fuel system on this bike is under pressure so petrol may squirt out at you as you cut the hose! You’re best off clamping the hose near to where it leaves the tank with a proper hose clamp before starting this job.
  • The internal diameter of the hose will fit either 6mm or 8mm nozzles on a filter, I have chosen one with 6mm nozzles and have observed no leaks after 120 miles of riding.
  • The external diameter of the hose is about 10mm, and the spring clips I have used either side of the filter are for a 10mm hose. They compress the hose around the nozzles because the external diameter of the hose expands slightly over the fittings. If using a filter with 8mm fittings, you may need larger clips, perhaps 11-12mm diameter ones.
  • You have about 40-50mm of usable space in the fuel hose for a fuel filter, including its nozzles, but you will have to kink/bend the hose out of shape if fitting a larger/longer filter. This may well over-stress the hose and/or disrupt the flow of fuel, starving the engine, so I do not recommend fitting a filter that is too long.
  • Blow through the new filter with your mouth to check its flow – it should be easy to blow through it, feeling a little resistance. Some filters are blocked when they leave the factory, believe it or not!
  • Look for an arrow on the filter that indicates the correct direction of flow for fuel, and install it the right way around.
  • Don’t forget to inspect/clean the filter at service intervals, or replace it if it’s a disposable one (I’d say after 7’500 miles or if it’s damaged/excessively clogged).
  • If you’ve fitted a plastic filter, inspect it after each ride for a few times after you first fit it, looking specifically for melting/warping of the plastic, due to the close proximity of the cylinder head, which gets very hot. It is not unheard of for plastic fuel filters to be damaged by heat, so be vigilant – we don’t want to be going up in a ball of flames now, do we?
I have observed no issues with the filter I have installed – no fuel starvation, no warping due to heat, no loosening of the fittings and no leaks. This included riding the bike in 30 degrees Celsius for 30 miles, mostly on dual carriageways, cruising at 60mph, followed by start-stop traffic jams for a mile or so at the end of the journey (where the engine would be most likely to get really hot).
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8 thoughts on “In-line fuel filter…

  1. Interesting job!
    Mine doesn´t have the fuel filter either (I think)… I guess that all kind of undesired stuff get in to my carb, so I think that is a good idea to add a filter… Although, I must study the situation first, the carburated one doesn`t have a fuel pump, and the filter may block the fuel flow or at least slow it…
    Before that, I need to look if it really doesn´t have the famous filter, because the fuel valve has some kind of removable tiny metal device that I really don`t have the idea of what it is… The owners manual doesn´t say anything about it…
    Greetings!

  2. Hi,
    I know my question looks a little bit stupid, but are you sure that you have put your filter on the right hose (because this one is the smallest (diameter) of the two ?
    I ask you that question because I also want to put an in-line fuel filter on a CBF 125.
    Kind regards.

    • Definitely the correct hose. The other hose is a vapour return hose, or breather that prevents too much pressure building up in the fuel pump (petrol can expand/evaporate quite easily over a small range of temperatures).

  3. Hello Buddy,
    I bought my CBF125 2 months ago and your site was a useful resource prior to buying the bike and even now that I have started carrying out maintenance.

    Can you please send me a message with the details of the In line filter you bought so Im sure Im buying the right one. Any indication of cost and where you bought it would also be useful.

    Kind Regards

  4. Great blog!
    I would also appreciate it if you could email me the details of the filter.
    I have seen people putting in line Bosch filters (BOSCH 1 687 432 005) but can’t find it online.
    Thanks in advance

  5. I bought the filter above but I don’t want to fit it myself as I do not want to do any damage (I’m not knowledgeable enough).
    So I took it to the service garage where my bike usually goes for servicing. I asked them to fit it in for me but the technician said that it can only cause problems as the pump, pumps fuel at high pressures and a filter may disrupt the flow. He advised me not to do it and I am torn.

    Can someone help me?

    • I don’t know of anyone who’s had a problem on a CBF125 from doing this. Personally, I have ridden over 25’000 miles with a fuel filter fitted and not encountered an issue at all. It sits on the line that feeds the pump from the tank, which should be gravity fed anyway. The pump would only ‘pump’ to the feed going to the injector to maintain a constant pressure at that point.

      • Thanks a ton for your quick reply.

        I’ve had the kangaroo problem twice now. First time they changed my pump but it happened again.

        I’m waiting for the new pump to arrive. They will also change my reservoir and also they will insulate it to prevent the heat from the engine causing fuel to vaporize.

        It makes sense that this problem that a lot of CBF 125 owners have may be caused by vapor lock.

        Let’s hope.

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