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Honda Cb250 running pod airfilters

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  • Honda Cb250 running pod airfilters

    Hi, I have a CB250 Brat with airbox removed and running K&N style pod filters (not as pictured). I realise it isn't how they were made and I will have less than perfect running, but has anyone found a good solution to the problems you get when not running the airbox?
    The previous owner has cut some of the rubber on the connecting boot so it clears the secondary? intake slot on the carb.
    If I replace the pods with new uncut ones as I intend will it cause more problems by blocking those?
    I have also been looking at trumpets and sock style filters, any feedback on those would be great too.
    I realise everything will be a compromise as the airbox is no longer being used but any feedback would be great.
    Mike
    Picture below shows the bike with trumpets which the seller said made the bike run badly so maybe not a good idea??

  • #2
    This will change your air fuel mixture you are probably running leaner on pods, I would keep an eye on the colour of your spark plug after making the change it would probably be much lighter but if it runs ok and the spark plug looks good you might have gotten away with it. Dont have much experience with four strokes but with two strokes it can be a compete nightmare with mods like this you need the bike to have the right mixture through the rev range.

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    • #3
      I'm afraid there's no two ways about it, you will be running weak. As a minimum I would raise the needle a notch or two and go up a couple on the main jet.
      Running bellmouths (trumpets) the same length as the stock carb to airbox rubbers with sock filters on would probably be a good idea as inlet tract and exhaust length are all designed to work together with gas harmonics through the engine.
      Your best bet would be to try and find somebody who makes a jet kit to run without the stock airbox as I seem to remember that old style CV carbs were not the easiest to rejet.

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      • #4

        The needle height/taper, main jet and slide cutaway all effect air/fuel mixture in differing amounts through the rev range.

        As Blue suggested raising the needle a notch should help adding more fuel to the extra air from the pods but you should still do a plug chop and for a period after check your plugs, two stroke bike owners probably take more care engines run hotter with a higher risk of seizing.

        When you do a plug chop a spark plug with a narrow slot cut away to see the colour on the insulator across the full length helps but not required, its not easy to see down into a spark plug. Your just going to ride the bike maintaining different revs for periods of time and inspect colour.

        I have added a link its for two strokes but has a nice chart showing how various parts of the carb effect the fuel mixture in different throttle range.
        Last edited by nortonj; 9th-01-2018, 06:34 AM.

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        • #5
          Like the other guys say just rejet the carbs and job done. It's not as difficult or scary as it sounds and you can give em a little clean while you're at it..........
          On my builds i always junk the old airbox in favour of pods, just realise you are now getting in more air than before so a larger jet to let in more fuel will balance things out as before. I just rejetted a Honda Cx500 for a guy running pods, standard jets were, Primary jet 78, secondary jet 112. I upped it to primary 90 and secondary 120. Now runs sweet as a nut..............But it's always worth running the bike to see how it goes first as i have had bikes ive put pods on and they ran sweet without rejetting.
          Rejetting sizes are a little trial and error but those are the numbers i always use on CX's maybe google it for your cb250 see what the original sizes are then what other people are saying they use with pods. K&N pods are one of the best so don't change them just sort your jetting! Then a little twisting of your air/fuel screw should get it about perfect....
          wemoto.com sell them or davidsilverspares.co.uk
          Last edited by wisp; 10th-01-2018, 06:37 PM.

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          • #6
            Thanks all, will get them jetted correctly for sure.
            One further question, are Keihin jets the same across all their carbs? IE a 105 jet will fit all of the range or do I need ones specific to the VB30b carbs I have?
            Mike

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            • #7
              Go to wemoto.com website, put in honda cb250, click on the model/year you have, scroll down to Fuel section, find carbuetta jet it gives you a menu of sizes pick the size you want and add to cart etc.
              I think they are about 3 quid and if youve selected the correct model/year it will be perfect...

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