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Thread: idle speed issues, /5 - worn advance springs?

  1. #1
    stucknarut
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    idle speed issues, /5 - worn advance springs?

    Question on my /5 resto. I've been having a hard time keeping idle speed down when warm. Running one cylinder at a time I set each side individually to ~500RPM, but running both together it goes to around 2000-2500. Carbs, timing, and points are set correctly, afaik. Worn-out advance springs? Other possibilities?

  2. #2
    Registered User mneblett's Avatar
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    When warm, is the idle ever in the 1000 range? I.e., does the rpm hang up around 2000-2500 after you pull in the clutch from running at speed, or is it *always* that high.

    If the rpm "hangs up" when coming down from a higher level, that is a sign the advance weights are sticking, and need to be cleaned/lubed.

    If the rpm is always high when the bike is warm, I suspect the base idle is just set too high, and you need to gradually reset the throttle stop screws and idle jet screws to bring the warm idle back down to where you want it. My wife's R75/5 would do the same thing, and that's how I addressed it.

    FWIW: Are you sure the engine is *fully* warm when you do the 500 rpm L/R setting? You symptoms would fit perfectly with a one-side-at-a-time adjustment when the carbs are not at operating temp (remember, the cylinders come up to temp quite a while before the airflow across the cylinders fully warms the carb bodies).

    HTH!
    Mark Neblett
    Fairfax, VA
    #32806

  3. #3
    stucknarut
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    Nope, the idle is always high when warm, and the throttle stop screw on the right is all the way out, with the idle mix screw almost all the way in. There's not much adjustment left, unless I run the idle mix screw quite a ways out (2-3 turns, compared to the book's starting setting of 3/4 turn out). It doesn't hang and then drop, it just runs high. For the last adjustment I rode the bike for about 50 miles on a warm day, so pretty sure it was thoroughly warm. With each cylinder at 490-500rpm, it immediately spun up to 2300-2500 with both.

  4. #4
    moosehead
    Guest
    Check to see if your throttle cables are getting hung up some where.

    Are they returning to throttle stop setting when you let off the twist throttle? With bike off, open up the throttle twist grip wide open then let it go...see if your throttles return to throttle stop position. Could be cables are hanging up either due to hang up in routing of the cables or also inspect cabes at carb end to see if any kinks or cable strand is starting to fray. One little cable strand outside the bundle can hang up that throttle very easily.

    Another check...do you have a throttle stop screw? Check to see if it is engaged or not. Make sure its off (screwed out)...don't ask why I suggest this...

  5. #5
    Registered User lmo1131's Avatar
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    You mentioned that the carbs are "set correctly", but have you taken these carbs all the way down, soaked them, and rebuilt them?

    There is an o-ring that is on the throttle-butterfly shaft. The shaft is secured by the bracket that anchors the throttle spring. IF that o-ring was in anyway displaced while reassembling, or if it has gotten "gummy" from age, then the shaft will not rotate freely.

    It took me three tries before I realized what was going on when I rebuilt my /5 carbs; I had gotten one of the o-rings twisted a bit as I pushed the shaft into it's bore; it was putting a lot of drag on the shaft. A tiny dab of lithium grease solved the problem; the o-ring slipped into the bore easier, and the shaft rotated freely.

    You might be able to simply remove the spring(s) and cable(s) to check if the shaft is rotating freely, but it's easier if you just pull the carburetor off. If you find resistance, you'll have to pull the two screws out of the shaft and remove the butterfly(s) to replace the o-ring(s). At which point you might as well replace all of the rubber bits.
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    Lew Morris
    1973 R75/5 - original owner
    1963 Dnepr

  6. #6
    stucknarut
    Guest
    Thanks for the tips. I'm pretty sure those aren't the issues. I completely tore down and rebuilt the carbs, and put a dab of silicon grease on the throttle o-rings. Before re-assembly both were working well and snapping closed. I did notice that the right side butterfly wasn't perfectly fitted, a bit of light around the edge, which is consistent with the throttle screw settings, but not so much that it should cause the high idle. Net, the throttles and throttle cables are working smoothly and not hanging.

    If a hanging throttle, worn cable, or vacuum leak, why would I only see the issue when both sides run together and not when setting independently? What would cause a difference when running together?

  7. #7
    Registered User 88bmwjeff's Avatar
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    Duane Ausherman has a bit of information on the advance unit and suggestions on how to test if the high idle is due to worn springs. I hope it helps.

    http://w6rec.com/duane/bmw/advance/index.htm
    Jeff in W.C.
    1988 R100 RT (the other woman)
    "I got my motorcycle jacket but I'm walking all the time." Joe Strummer

  8. #8
    stucknarut
    Guest
    Yep, that's the page that made me think worn advance springs, just wanted to get opinions based on my symptoms. Ordered new springs, hopefully it helps. If not I'll go buy a sixpack and go looking for a local airhead guru.

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