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Thread: R75/5 leaking bing carburetor

  1. #1

    R75/5 leaking bing carburetor

    Just got the bike and one of the carbs tends to leak. It happens anywhere between 5 minutes and several hours after I turn off the bike (with petcocks off). Shaking the bike a little bit tends to trigger it.

    I know there has been several posts on leaking bing carbs (I've read them all) and I've gone through snowbums writeup on the bing CV dance.

    https://bmwmotorcycletech.info/Carbu...-that-piss.htm

    The carbs were rebuilt 3 years ago with new floats and needles but the bike has been sitting for a long time so I figured it was probably just dirt in the needle seat. However, flushing it didn't solve the issue. With petcocks open I can easily completely stop the fuel flow by lifting the float with my finger. The floats also seem to be adjusted correctly with the top of the float aligning with the carburetor body at the moment fuel starts flowing (I've attached two pics, one where fuel flows and one where it doesn't).

    Measuring the fuel in the bowl after letting it sit for a little while with open petcock I measure around 16mm which seems to be a bit low. I don't see how low fuel could cause a leak though.

    I've also tried placing the full bowl on a paper towel for a while to make sure there's no leak from the overflow pipe and there isn't.

    I'm out of ideas. I guess the float might get stuck somehow but I don't see how that could start leaking hours after I turn off the bike.

    Any thoughts would be appreciated.


    IMG_20190825_190647.jpgIMG_20190825_190713.jpg

  2. #2
    Liaison 20774's Avatar
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    Welcome to the forum! Glad you found Snowmbum's website. Hope your eyes are OK after all the reading.

    Sounds like you've done everything right in terms of troubleshooting. Low fuel in the bowl isn't right...some adjustment is needed.

    Have you rebuilt the petcocks? They shouldn't pass fuel when off. If they do and fuel gets into the cylinders, you can get hydro lock and bend things.

    Floats look a bit yellow. Might be time for a fresh set.
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  3. #3
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    I had exactly the same problem with my 75/5. Carbs rebuilt, checking float to see if fuel flow stops etc.
    It turned out to be a leaking pet cock. I did not notice it by just pulling the fuel hose off. It took a while
    before the first drop formed at the outlet but is was enough to cause the carb to leak eventually.

  4. #4
    #4869 DennisDarrow's Avatar
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    Sounds like you have checked a lot of reasons for the leakage. By chance is the carb that leaks on the left side? Are you parking the bike on the sidestand? When the leakage is happening, and then you take off the bowl, how full is it?

    By the way, 22 to 26 mm is a good depth for the fuel to be in the bowl. Measure from the center of the ring in the middle of the bowl. Those floats are pretty dirty looking. It might be a good idea to get some new ones. The floats tend to soak up fuel over time. This causes them to get heavy and then there is an overflow condition because they ride lower in the bowl and cause it to fill up......Good luck.....Dennis

  5. #5
    Quote Originally Posted by eurotourer View Post
    I had exactly the same problem with my 75/5. Carbs rebuilt, checking float to see if fuel flow stops etc.
    It turned out to be a leaking pet cock. I did not notice it by just pulling the fuel hose off. It took a while
    before the first drop formed at the outlet but is was enough to cause the carb to leak eventually.
    Thanks!

    I've tried leaving the carb bowl off with the petcock open to see if the petcock leaks, but I no fuel was coming through. I guess it could be that I wasn't patient enough. I'll try it again placing an empty bowl beneath, and leaving like that for a couple of hours, to see if the petcock leaks.

  6. #6
    Quote Originally Posted by DennisDarrow View Post
    Sounds like you have checked a lot of reasons for the leakage. By chance is the carb that leaks on the left side? Are you parking the bike on the sidestand? When the leakage is happening, and then you take off the bowl, how full is it?

    By the way, 22 to 26 mm is a good depth for the fuel to be in the bowl. Measure from the center of the ring in the middle of the bowl. Those floats are pretty dirty looking. It might be a good idea to get some new ones. The floats tend to soak up fuel over time. This causes them to get heavy and then there is an overflow condition because they ride lower in the bowl and cause it to fill up......Good luck.....Dennis
    Thanks for the reply.

    The carb that leaks is on the right side and I have a sidecar attached so the bike is level. When I measured the gas level in the bowl I wasn't measuring in the center of the divot. That probably adds a couple of mm so I'm guessing it's around 18mm, still a good ways below what you say it should be.

    I haven't done a float test yet so I'll do that (or maybe I'll just go ahead and order new floats and needles, probably worth it just to be safe).

  7. #7
    Quote Originally Posted by 20774 View Post
    Welcome to the forum! Glad you found Snowmbum's website. Hope your eyes are OK after all the reading.

    Sounds like you've done everything right in terms of troubleshooting. Low fuel in the bowl isn't right...some adjustment is needed.

    Have you rebuilt the petcocks? They shouldn't pass fuel when off. If they do and fuel gets into the cylinders, you can get hydro lock and bend things.

    Floats look a bit yellow. Might be time for a fresh set.

    Thanks. I've already spent hours on snowbum's website (also had a separate starter problem that I've since figured out).

    I'll check the petcock. Thanks for the advice.

  8. #8
    #4869 DennisDarrow's Avatar
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    Quote Originally Posted by jakobw View Post
    Thanks for the reply.

    The carb that leaks is on the right side and I have a sidecar attached so the bike is level. When I measured the gas level in the bowl I wasn't measuring in the center of the divot. That probably adds a couple of mm so I'm guessing it's around 18mm, still a good ways below what you say it should be.

    I haven't done a float test yet so I'll do that (or maybe I'll just go ahead and order new floats and needles, probably worth it just to be safe).
    When you get those new floats, perhaps you might set the old ones aside to use on a rotation basis. I am sure that when you take off the existing floats and hold them in your palms....One set in one hand and the others in your other hand. There will probably be a significant difference....Set the "old" ones aside. They will DRY out over time and the next time you need to change floats, you will have a pair that is "dry" and ready to change out....Just thoughts....

  9. #9
    You guys were right, it was the petcock. I didn't find it the first time I tested it because the gas dripped onto the brake pedal and travelled all the way along the lever before dripping to the ground, so it missed the bucket with paper towel I had placed under the carb to check for leaks.

    Thanks for all of the good advice.
    1973 R75/5

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