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Thread: 76 R90/6 - right carb leak / weird 'gyrating' sound at speed

  1. #1

    76 R90/6 - right carb leak / weird 'gyrating' sound at speed

    took the beemer camping this weekend with a couple of buddies in upstate NY, about 300 miles round trip. on the ride home, i found while riding in 4th and 5th gear at over 4,000 RPM i was hearing a strange almost gyrating sound, but really could not hear where it was coming from. bike was riding completely fine, no observed changes in power/handling/shifting/etc. when i got home, checked all fluids and saw that all levels were good, but i had a small leak coming from the right carb. haven't torn into it yet but any thoughts on what it could be? i took a look at snowbum's page (https://bmwmotorcycletech.info/Carbu...-that-piss.htm) as a starting point.

    as far as the gyrating sound, i should also note that i'm riding with a new-to-me model of helmet that has a flip up face shield where in the past my helmet had no shield. so it really could just be wind sound.....but can't be 100% of it.




  2. #2
    Liaison 20774's Avatar
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    Where does the leak seem to be starting at? Have you checked the float heights in the carbs?

    As for the helmet, seems like it would be easy to do some comparisons. See if you can borrow a helmet. Ride without a helmet (maybe some goggles). Retry your new helmet. You should be able to isolate it if it is the helmet.
    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
    leak is observed right where the intake boot meets the carb. you can see the drip on the hose clamp where the boot joins to the carburetor.

  4. #4
    Liaison 20774's Avatar
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    Sounds like a stuck float, grit on the seat, or float level set too high, resulting in too much fuel in the bowl.
    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
    If it's not the new helmet, and the noise is coming from the bike (near the right, topside of the engine?), it could still have the OEM disc-type crankcase vent. Replace it with the later reed-type crankcase vent, and that will likely resolve the "gyrating" (AKA: "turkey gobble") noise.

  6. #6
    B Reams brook.reams's Avatar
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    Quote Originally Posted by khittner View Post
    If it's not the new helmet, and the noise is coming from the bike (near the right, topside of the engine?), it could still have the OEM disc-type crankcase vent. Replace it with the later reed-type crankcase vent, and that will likely resolve the "gyrating" (AKA: "turkey gobble") noise.
    Per khittner's suggestion of the cause, is the leak oil or gasoline? Oil leaking there suggests the crankcase breather valve (aka crankcase vent) is puking oil as it feeds only the right side carburetor intake on that year bike.

    I hope this helps.

    Best.
    Brook Reams.
    Brook Reams - Arvada, CO
    Endeavor to Persevere
    Various Two-wheeled Vices, All BMW || Website: Airhead Rebuild Projects
    2004 R1150-RS||2002 F650-GS||1983 R100RS||1977 R100RS||1973 R75/5

  7. #7
    '99 '03 '06 National Co-Rally Chair Friedle's Avatar
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    If you are not sure if it is leaking oil or gas, just smell your boot. The leak may also be gas from a worn fuel line to the carb.

    Friedle
    Ride fast safely

  8. #8
    Quote Originally Posted by brook.reams View Post
    Per khittner's suggestion of the cause, is the leak oil or gasoline? Oil leaking there suggests the crankcase breather valve (aka crankcase vent) is puking oil as it feeds only the right side carburetor intake on that year bike.

    I hope this helps.

    Best.
    Brook Reams.
    update: it's oil. there is oil all on the bottom side of the intake boot going into the right carb. is there a thread regarding repairing the crankcase breather valve?

  9. #9
    Liaison 20774's Avatar
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    It's relatively straight forward. Remove the engine top cover. Find old breather. If still the old style, you'll need to remove the round chip covering the hole. Then you'll need to fashion some kind of puller to knock the old breather up-and-out. Then install the new breather, with proper orientation. Reassemble the hoses. Be sure and check the small hole just forward of the breather if it is in your engine case. Some older threads about the subject here:

    https://forums.bmwmoa.org/showthread...nt-Maintenance

    https://forums.bmwmoa.org/showthread...kcase-Breather

    https://forums.bmwmoa.org/showthread...eather-removal
    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!

  10. #10
    thanks kurt. another follow up question:

    is this a project that can wait until the winter? we probably have three or four more solid weeks of good riding in NY and again i didn't feel that it affected the performance of the motor. seems like an easy enough repair. just want to be sure i've got nothing catastrophic on my hands as long as i'm monitoring my fluid levels.

  11. #11
    Liaison 20774's Avatar
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    I would think you could wait, as long as you can stand the "gyrating" sound and the occasional drips of oil. If the oil coming out of the breather is copious, then it could foul the spark plug but it would have to be really bad and you would probably see other symptoms like smoke coming out of the right muffler.
    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!

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