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Help XS 250 air filters

Discussion in 'Yamaha 250cc Twins' started by EBB, May 8, 2020.

  1. EBB

    EBB Member

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    Hi guys,
    Just joined this forum for help and advice.
    A 1980 xs 250, US custom UK model has been dumped on me with the challenge of getting it up and running.
    1st off, its got no air box.
    What after market filters can l use and what jet size do I need?
    I'm sure this has already been covered but I'm partially sighted and not to good at finding things.
    Any pointers would be great.
    Thanks
     
  2. 2valve

    2valve Well-Known Member

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    Hi. It would be much easier for you to purchase another airbox / new air filter / air tubes / clamps etc than wasting your time on working out what jet's you need. The original carbies are the CV type , so the air box etc will be needed anyway. Have enclosed some links below for you below to look over. Is this the same looking bike you have though ?. If your lucky enough the seller will have every thing you need. Ask the seller to send some pictures so you can work out what part's you have missing and to make sure that the part's are correct for your year model just in case it's been played with.

    https://www.ebay.com.au/itm/Yamaha-...713098?hash=item33ec97f28a:g:dlwAAOSwvjBbHPgL

    https://www.ebay.com.au/itm/Yamaha-...593590?hash=item341f9ba5b6:g:BiQAAOSwRgJXlmAG

    https://www.ebay.com.au/itm/Yamaha-...706710?hash=item33cbcf8416:g:NHUAAOSwsN9XAFBx
     
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  3. EBB

    EBB Member

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    Many thanks for your reply,
    What I didn't say was this thing begin chopped about to resemble a flat tracker ( which I must admit looks pretty good)
    Hence its gotta have K&n style filters.
    Thanks
    EBB
     
  4. 2valve

    2valve Well-Known Member

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    No problems , happy to help.Not to be rude here , but is are all of the air box mounting lugs still in place. It's amazing what some people do.Any pictures of the bike would be useful also.
     
  5. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

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    Well good luck with using K&N on CV type carbs. Most who have tried have spent untold hours and dollars (pounds) on trying to get the carbs to run right before giving up and selling their bikes. The best help I can give is to make some long intake trumpets with the filters on the ends. This will help (a little) to smooth the airflow into the carb mouth. A possibly cheaper option (as Alan Millyard did on his six cylinder FZR engine) is to buy a couple of cheap Chinese made slide carbs and filters and replace the CV carbs.
    What is the intended use of the finished bike. If it is only to ride to a cafe and back then the jetting may be ok. As always with carbs, try it and see.
     
  6. Frankster

    Frankster Grey Pride...Adventure before Dementia Staff Member Premium Member Ride and Events Crew

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    Your issue with pod filters is they have a certain (high) level of restriction that works against you and tuning the engine. As Murdo said, if you insert an inlet tube between the carb and pod filter, then you give the carb a bit of additional space to suck in air. It's almost impossible to jet a CV carb for Pod filters as the engine will work fine at idle or low revs (idle jet circuit), but run rich once the revs rise and the amount of air being sucked in is restricted (main jet circuit). The only real way to jet the carbs for pods is to reduce the size of the main jets to properly meter fuel and air at higher revs. To do this, you will have to go down in jet size and that will give you poor acceleration and reduced power. In short, pods look good, but they're awful from a practicality sense. Bell mouth inlets or velocity stacks are way better and easier to tune, but the lack of a filter means bad things can get sucked into the engine. In closing, pod filters look good from afar, but they're far from good!
     
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    Last edited: May 10, 2020
  7. EBB

    EBB Member

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    Thank you all,
    I've got some head scratching to do.
    I've gotta get it running first.
    That'll be on open carbs.
    Trouble you stock parts is it's easier to find a unicorn here in the UK !!
     
  8. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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  9. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

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    It's an XS twin, not the SR single.
     
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  10. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    Sorry my mistake

    There's a few genuine new airbox / filter parts on ebay

    The airbox/case was also used on the XS400 Part number 3X8-14401-00


    https://www.ebay.com.au/itm/Yamaha-...636604?hash=item523487d9bc:g:looAAOSwjoFZrVLP

    Filter in UK
    https://www.ebay.com.au/itm/NOS-Gen...292162&hash=item340cfeb43e:g:Qg0AAOSwZexdCOrs

    https://www.ebay.com.au/sch/i.html?...efLoc=2&_osacat=0&_odkw=3X8-14451-00-00&rt=nc

    Nos side cover
    https://www.ebay.com.au/itm/Yamaha-...923291?hash=item5da824819b:g:fysAAOSww9Bcpdji


    You could try places like boats.net, megazip.net, en.impex-jp.com etc and search using the original part number's
     
  11. EBB

    EBB Member

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    Hi guy's,
    If I was to go down the Bell mouth route, which way do I jet, up or down ?
    Thanks
     
  12. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Last edited: May 11, 2020
  13. Frankster

    Frankster Grey Pride...Adventure before Dementia Staff Member Premium Member Ride and Events Crew

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    Neither to start with. Just run the bike as is and test ride it to gauge performance during acceleration, cruising, high revs etc. Once you have that information, you/we can go from there. Your bike should idle fine even with pod filters. Without an airbox, your carbs will suck in hot air when you're stationary and colder air when moving, which may also have an affect on how the bike runs. The bell mouth or velocity stack (by its design) will condense the air being sucked into the carb, so that may have a slight affect on acceleration/performance. Blair's suggestion to go VM carbs is also an option, but be aware that a restricted air flow is always going to be a restricted air flow. Even if you went to EFI, you're still limiting the amount of air by using a (small) pod filter. The kit Blair has linked looks pretty good as it has a larger pod filter. Even without knowing the exact flow rate of the filter material in those pods, you'd expect them to be a better option than cheap Chinesium rubbish.
     
  14. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

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    Try it first and see.
     
  15. EBB

    EBB Member

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    Thanks guys,
    You've given me plenty to work with.
    Thank you
    EBB
     
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  16. EBB

    EBB Member

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    On a slightly different note,
    I set the timing up today and got hear fired up on open carbs.
    I fully expected it to be a bit rough because of no filters.
    Idles OK but the slightest slidetest twist of the throttle and the rev's go though the roof and stay there.
    I've cleaned the carbs and checked the throttle returns.
    Is it just because I've got no filter on? (I haven't got one yet)
    Or is it the timing?
    Thanks
    EBB
     
  17. EBB

    EBB Member

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    I should add that this olde bus is far from road worthy at present and everything is being carried out in my garage.
     
  18. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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  19. Frankster

    Frankster Grey Pride...Adventure before Dementia Staff Member Premium Member Ride and Events Crew

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    Idles OK but the slightest slidetest twist of the throttle and the rev's go though the roof and stay there.

    If you put the bike in gear and use the clutch (load) to drag the revs down, do the revs rise again if you pull the clutch in or do they stay low? It could be a balance issue. What main jets were in the carbs?
     
  20. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Usually not settling to idle is caused by the idle mixture being too lean. What happens if you engage the choke a little?
     
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