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Thread: '77 r100rs low idle at times

  1. #1
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    '77 r100rs low idle at times

    About 1,000 miles since carbs were rebuilt by Bing, synced by my local/reputable airhead mechanic, idle set at about 800 rpm. When the engine gets real warm and I quickly let off the throttle, the rpm drop way down, below 500 rpm and I have to blip the throttle to keep it from stalling, the idle will then come back to 750-800 rpm and remains stable. Engine runs flawlessly otherwise. Any ideas why I get the drop and then return to normal idle with the warm engine?

  2. #2
    Administrator 20774's Avatar
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    First, I'd suggest you reset the idle to at least 1000 maybe even 1100...ensures good oil flow at idle...800 seems too low IMO. The next thing I would check is the height of the floats, or rather the height of the fuel in the float bowls when the floats shut off flow. You could have a lean situation which shows up when you chop the throttle.
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    '78 R100/7 & '69 R69S & '52 R25/2
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  3. #3
    Pistol Shooter headhog's Avatar
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    Quote Originally Posted by 20774 View Post
    First, I'd suggest you reset the idle to at least 1000 maybe even 1100...ensures good oil flow at idle...800 seems too low IMO. The next thing I would check is the height of the floats, or rather the height of the fuel in the float bowls when the floats shut off flow. You could have a lean situation which shows up when you chop the throttle.
    I concur with the above. I set my idle just to the + side of 1000. This provides adequate oil pressure/flow at idle. The next logical step is checking the float levels.
    MSF Instructor
    1992 R100GS
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    2007 Harley Electra Glide Classic

  4. #4
    Registered User toooldtocare's Avatar
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    I never trusted the tach anymore that I did the speedometer and had the same problems as you at 800 RPM, thus moved my bikes up to 1,000 to 1,000 range too.

    Wayne

  5. #5
    Registered User cpitale's Avatar
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    I had a similar situation on my 77 rs it was actually the aftermarket boyer ignition causing the idle to drop went back to a mechanical advance and its ran beautifully since

  6. #6
    One big Oaff brewmeister's Avatar
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    Hi guys, what condition is the mechanical advance unit in?? How are the springs? Is the mechanism free moving gunk free?

    A 77 r100/7 would not come down to idle smoothly because the advance unit was sticky and wouldn't always retract completely , the bike would idle with the advance ingauged causing rough,inconsistancely smooth idle.

    never the less,good luck.
    81 R100RT

  7. #7
    rsbeemer 22600's Avatar
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    Plus on the idle

    None of the 100rs I've had would idle good at 800 but at 900-1000 would idle fine if everything else was set up right. I've had mine idle at 800 before and it sure sounded pretty but when I came to stop after it warmed up good it would drop down under 800 and sound like it was gonna die. The 750's I owned would idle somewhere under 800rpm's, again if timing and all was good. So, try cranking it up some and see if that will help. Let us know what you find.

    DW
    1978 R100rs MOA#22600 125cc Kymco
    The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails.

  8. #8
    Registered User lmo1131's Avatar
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    I remember the first BMW motorcycle that I'd ever saw. It was an an old, somewhat derelict, R69s that a friend bought for his dad. Despite it's ragged appearance it ran like a top, and would sit there and idle happily at something under 800 rpm. With that gear-driven cam it seemed like all you could hear was the intake tract breathing against the exhaust.

    My R75 ... 1000-1100 rpm, with a slight tendency, depending on ambient temperature, to idle faster when coming off a long run on the freeway; it always returns to "normal" within a few minutes. Not quite as pleasing (or as impressive to motorheads) as that slow ticking over of the older machines but you need to keep the oil moving and the amps up.
    "It is what you discover, after you know it all, that counts." _ John Wooden

    Lew Morris
    1973 R75/5 - original owner

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