R100RS Lacking Power - Aftermarket Diaphragms!

Discussion in 'Airheads' started by jchussey, Jul 13, 2019.

  1. jchussey

    jchussey Adventurer

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    Hoping to receive some sage advice on an issue I'm having with my 1984 R100RS:

    I have about 800 miles on an extensive restoration, including having the heads done (valves, guides, springs, seats, Randy Long), New pistons (Wossner 9.5), new Bosch coils, complete Bing rebuild, new plugs, new fuel tank screens.

    The bike is sluggish. It starts immediately, idles well and accelerates smoothly, but feels pretty gutless. I have it jetted exactly the same as my 1984 R100CS, which has much better throttle response and noticeably better "grunt" when cracking the throttle open at 3k rpm. I have checked the timing and am able to see full advance at 3k rpm. I am positive the valves are adjusted correctly, and am very confident with my Bing rebuild and adjustment abilities.

    Of note, I did use aftermarket diaphragms, which I have heard mixed reviews on, and they do seem thicker than BMW oem. Could this be my issue? Has anyone here experienced similar diaphragm- related performance issues?

    Otherwise I am stumped.
    #1
  2. boxerboy81

    boxerboy81 Stay Horizontal

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    Reading from the start, new diaphragms incorrectly installed was my first thought. They can easily dislodge when re installing the carb top.
    I doubt the af/m ones would make that much difference though otherwise no one would ever use them?
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  3. ME 109

    ME 109 Long timer Supporter

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    Are the carb slides fitted correctly? The two tubes on the bottom of the slides have angled cuts, which face forward.
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  4. boxerboy81

    boxerboy81 Stay Horizontal

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    The view from the head.
    [​IMG]
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  5. Warin

    Warin Retired

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    Spark plug readings to see if it is running lean/rich... ? Unless you have a sensor for that...
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  6. Disston

    Disston ShadeTreeExpert

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    So you are comparing an '84 R100 CS to an '84 R100RS? This seems fair but are both bikes 32 or 40 mm carbs? Are the final drives the same?

    Just trying to eliminate things that would make these bikes not as similar as they sound.
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  7. jchussey

    jchussey Adventurer

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    Yes - both 40mm carbs/ big valve heads.
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  8. jchussey

    jchussey Adventurer

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    Spark plugs do seem remarkably clean after 800 miles. Not much color on them.
    #8
  9. jchussey

    jchussey Adventurer

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    #9
  10. Disston

    Disston ShadeTreeExpert

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    There were 2 questions in my querie. Are the final drives the same ration?

    I have to ask because I don't know if these bikes are as same as they sound.
    #10
  11. jchussey

    jchussey Adventurer

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    I am certain that the diaphragms are installed correctly. I did notice that when the alignment tabs are centered, the curve profile of the slide does not exactly match the curve profile of the carb body.
    #11
  12. jchussey

    jchussey Adventurer

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    Both 33/11
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  13. jchussey

    jchussey Adventurer

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    Slides are installed correctly.
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  14. theDQG

    theDQG Been here awhile

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    Check petcocks and tank screens.. might be debris in there? What is the condition of your gas tank interior?

    Smooth acceleration without power seems like a restricted fuel or air flow issue to me.
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  15. boxerboy81

    boxerboy81 Stay Horizontal

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    Dirty air filter? Intakes blocked, eg tank bag straps under the tank?
    #15
  16. tlub

    tlub Long timer Supporter

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    Do a compression check on both bikes (with carbs removed but I suspect you know that) and compare.
    Swap the carbs if that doesn't tell you anything.
    You may want to see if both are running the same throttle gear set in the twist grip. I believe there are different ones to get different responses.
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  17. SportsterTom

    SportsterTom Been here awhile

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    This engine might want more fuel than the CS. Try raising the needle one notch and check the results. Even though the engine is fresh do a hot cranking compression check to how the cylinders are sealing.
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  18. jchussey

    jchussey Adventurer

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    Thanks for everyone's help so far.

    These are Wossner pistons from Motoren Israel for the B cylinder, which is what is on my bike. This engine had 30k miles when I took the top end apart, and the crosshatch in the Nikasil looked fine. The engine doesn't smoke and is not using oil at a concerning rate.

    Bike warm, carbs removed, 125psi in each cylinder. Sounds low on the surface, BUT my understanding is that the calculation is: compression ratio plus one multiplied by atm (which is 12 here in Denver), so:

    (9.5 + 1) x 12 = 126psi


    My CS is all original with lower compression pistons and can eat this RS's lunch. I also have another cafe'd R100 that I put 9.5 Siebenrock pistons in and it rips. AS I mentioned previously, the heads were done by Randy Long, a very well regarded machinist, so I strongly doubt any valve-related issues, but I don't have a leakdown tester to verify.

    Next step is to swap carbs with my CS.

    Thanks again everybody
    #18
  19. Steve W.

    Steve W. Boxer Pilot

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    Might want to perform a leak down test. My ole 78S blows 160 cold with a snap on gauge. BTW that pressure depends a lot on cam timing and cam profile. Cheers
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  20. bmwrench

    bmwrench Long timer

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    I've seen a couple of references to Wossner pistons, but I can't find anything on their website. Are they an exclusive to Motoren Israel? Do they have the correct squish angle?
    #20