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Thread: 76 R90/6 - gas cap hiss after ride

  1. #1

    76 R90/6 - gas cap hiss after ride

    ok so. went out for about a 25 mile ride yesterday around the city, bike is running well, noticed a little bit of excess heat coming off the heads in top gear around 60-65mph, but other than that, pretty solid. tank is about half full.

    when i get back from the ride and put the bike on center stand and let it cool off a little bit. when i go to move it back into the garage and take it off the center stand, i get a strong hissing sound from the gas cap. if i pop the cap the hissing sound goes away.

    what the hell? should i be concerned? do i have to replace the entire cap or a gasket?

  2. #2
    Liaison 20774's Avatar
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    Sounds like your cap is not venting. Typically that shows up when on a ride and the bike seems starved for fuel for no apparent reason. In those situations, cracking the gas cap open will release the vacuum and let fuel flow.

    Next ride, try cracking the gas cap before you take off. Then upon return, see if the hissing is absent. But given your observation that the hissing stops when opening the cap, suggests that it's corroded inside and doesn't let it breathe.

    A little reading here:

    https://bmwmotorcycletech.info/fuel-caps-airheads-K.htm
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    Quote Originally Posted by 20774 View Post
    Sounds like your cap is not venting. Typically that shows up when on a ride and the bike seems starved for fuel for no apparent reason. In those situations, cracking the gas cap open will release the vacuum and let fuel flow.

    Next ride, try cracking the gas cap before you take off. Then upon return, see if the hissing is absent. But given your observation that the hissing stops when opening the cap, suggests that it's corroded inside and doesn't let it breathe.

    A little reading here:

    https://bmwmotorcycletech.info/fuel-caps-airheads-K.htm
    before taking off meaning:

    -while the bike is off center stand and not running, pop cap, and then start bike and go
    -while the bike is off center stand and running

  4. #4
    Liaison 20774's Avatar
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    I don't think the timing of loosening the cap is that critical. Don't completely open it but enough to ensure that air can get in/out...you don't want to let gas easily slosh out. Do you have the cap that screws into the tank or the older style with the clicking and pivoting gas cap?
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  5. #5
    click/pivot style.

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