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Thread: Gas cap venting, what to do?

  1. #1
    Barback King rapiddog's Avatar
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    Question Gas cap venting, what to do?

    1981 RS

    Been reading a lot of threads about the gas cap not venting but haven't found one that says what to do about it.
    I was riding yesterday and a couple timed while cruising at 3000rpm the bike started acting like it was running out of gas. This has been only one of the issues with the bike and I've addressed all the fuel and ignition issues. Complete new ignition, rebuilt Bings etc etc etc.
    So, I tried unscrewing the gas cap while cruising along and it cured the problem, several times in fact.

    So how do I fix the gas cap? Replace it?
    I'm certain this is the issue. I noticed pressure build up when I had stopped for gas at the pump when releasing the cap like on a car.
    I wonder if this ay be some of the reason I get a lot of popping when down shifting...?
    "...the burble of my exhaust unwound like a long cord behind me. Soon my speed snapped it, and I heard only the cry of the wind...." Larry of Arubia

  2. #2
    Administrator 20774's Avatar
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    Snowbum has written about where to drill the cap. That was also the subject of a thread a while back.
    Kurt -- Forum Administrator ---> 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
    Mike V. #30064 30064's Avatar
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    Pretty simple job. I think Bum is using my photos I sent him on his website, the photos are self explanatory. If I had it to do over again, I would have used a smaller drill bit for the hole. Keep in mind the cap will no longer seal fuel as it once did in the event your bike falls on its side. I was having terrible trouble with no fuel flow on extended runs due to the lack of venting of my cap on my 78 R100/7. The drilling fixed it and I no longer pull a vacuum in my tank while touring just prior to that "regular" to "reserve" transition. My problem was compounded because I tour with a tank bag. The bike would just flat starve and I would have to pull to the shoulder and remove the cap to resume flow. Very dangerous scenerio.

    Find the center of your cap, measure 1/2" radially (arrow in picture), use a center punch and drill through the shell carefully. You'll notice a void (air space) once past the outer shell, continue past the void and through the soft matter about 1/8" thick and stop. Clean up the interior of the cap the best you can. Bum explans it in detail on his web site. It works. Here's Bum's link.

    http://bmwmotorcycletech.info/fuelfltrs&petcocks.htm

    Mike V.
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    Mike V. / San Diego
    MOA#30064 / ABC#11027
    78 R100/7 [orig. owner] / 81 R65 [restored]
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  4. #4
    Barback King rapiddog's Avatar
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    ..done, found that linky. fingers crossed...
    "...the burble of my exhaust unwound like a long cord behind me. Soon my speed snapped it, and I heard only the cry of the wind...." Larry of Arubia

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