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Thread: Hesitation at high RPMs

  1. #1
    BRACKRAT
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    Hesitation at high RPMs

    So my trusty 1980 R100T is running like a champ until 5th gear around 4400 rpms. At that point no matter the throttle, she seems to really stutter and decrease slowly in speed. My dad suggested the condenser might be going bad, so I threw in a new one along with new points while I was at it. No such luck. Any ideas?

  2. #2
    Liaison 20774's Avatar
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    Are you getting enough flow through the petcocks? Minimum flow should be over 300cc per minute or 5 to 6cc per second.

    Are the floats set too lean? Should be about 22-24mm of gas in the float bowl when the float shuts off flow.

    Is the gas cap vent blocked such that you are creating a vacuum in the tank? Next time it happens, reach down and crack the gas cap. Wait a few seconds and see if the problem disappears.
    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
    #4869 DennisDarrow's Avatar
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    Agree with 20774 or could be a diaghram.....pardon spelling.................is this just a sudden occurence???????...............what other work have you done just before this started????

    Anyway to reach down when this is happening and give a slight pull on the throttle cable to tell you which side is the offendor????

    Good Luck.........Dennis

  4. #4
    BRACKRAT
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    Quote Originally Posted by DennisDarrow View Post
    Agree with 20774 or could be a diaghram.....pardon spelling.................is this just a sudden occurence???????...............what other work have you done just before this started????

    Anyway to reach down when this is happening and give a slight pull on the throttle cable to tell you which side is the offendor????

    Good Luck.........Dennis
    I managed to get off work a few hours early last wednesday and went for a two hour ride. She ran great until the final stretch home when the problem presented itself.

    Recent work has included a clutch plate replacement, and some regular maintenance.

    I will try finding the troubled float the next time I ride, as well as the gas cap trick and report back.

    Thanks

  5. #5
    BRACKRAT
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    Quote Originally Posted by PT9766 View Post
    I had this symptom on a '78 R100 - didn't want to pull 5th at 5000 rpm but would rev OK above that when not under load. Bike was not "getting enough gas" although the fuel lines and carbs checked out OK.

    But in looking at it, it dawned on me that the carb-to-head rubber hoses were likely the originals and if so were probably a bit porous and leaking air under suction at load. Got new hoses and in removing the old ones found they were hard and brittle, especially on the head end. This single change solved the reving problem at speed and also resulted in more consistent idling and smoother off-idle performance.

    PT9766
    Another good idea. The symptoms seem the same, and I'm sure the hoses haven't been replaced. This would explain the problem only presenting itself at such a high speed, as well as some erratic idling i.e. high idle (1800-2000) when hot.
    When you experienced the problem on your 78, did you have any acceleration problems? The reason I ask is I don't have any problem. She'll really take off if she needs to.
    Thanks!

  6. #6
    Liaison 20774's Avatar
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    Quote Originally Posted by brackrat View Post
    Another good idea. The symptoms seem the same, and I'm sure the hoses haven't been replaced. This would explain the problem only presenting itself at such a high speed, as well as some erratic idling i.e. high idle (1800-2000) when hot.
    Spray some carb cleaner all around the tube when the bike is idling. If there are cracks/gaps, the cleaner will get sucked into the intake and change the idle speed. That will let you know the tubes are definitely cracked. Or just replace them because...
    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!

  7. #7
    Bill Burke
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    Recent clutch work: clutch cable out of adjustment - slipping at speed?

  8. #8
    Registered User lkchris's Avatar
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    Quote Originally Posted by brackrat View Post
    I managed to get off work a few hours early last wednesday and went for a two hour ride. She ran great until the final stretch home when the problem presented itself.

    Recent work has included a clutch plate replacement, and some regular maintenance.

    I will try finding the troubled float the next time I ride, as well as the gas cap trick and report back.

    Thanks
    You really should inspect carb diaphragms.

    Diaphragm failure is very common and replacement every two years is recommended.
    Kent Christensen
    21482
    '12 R1200RT, '02 R1100S, '84 R80G/S

  9. #9
    RAYGUN
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    Where did you find the tubes between the carb and head? I looked at http://www.realoem.com/bmw/select.do?vin=&kind=M&arch=0 but found none.

  10. #10
    Liaison 20774's Avatar
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    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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