'89 r100gs surging at speed

Discussion in 'Airheads' started by OZbeemer, Jul 14, 2012.

  1. OZbeemer

    OZbeemer Been here awhile

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    After a long rest I got the gs back on the road recently.

    Went through the usual clean and reset etc. Now the bike seems to surge when held at a constant throttle position say 45- 65 mph.

    Bike starts great, idles great once warm, and no problems coming off of idle. Pulls strong but the issue shows up while maintaing a constant speed.

    Carbs are cleaned, diaphragms look good, valves are maybe a touch loose and will address that and a retorque once its cooled off overnight. Carbs are synched right I think ( first time using a twinmax).

    What are the thoughts of the masses?

    Thanks
    #1
  2. Solo Lobo

    Solo Lobo airhead or nothing

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    Is the surging something that started after you changed something, or has it always been the case with this bike?

    If always, check the needle positions...
    #2
  3. OZbeemer

    OZbeemer Been here awhile

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    Hard to say when it started due to the bike being laid up for awhile, but I will say that I don't recall it doing anything like this when I put it away.

    I will check the needle positions to make sure they are the same, but they should be as I have not moved them since I rebuilt the carbs years ago.
    #3
  4. ignatz72

    ignatz72 call me iggy

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    If the only thing that's changed is storage time, I'd recommend looking at all the rubber bits in the carb circuit - o-rings and diaphragms. These may "look" fine but are cheap to replace.
    #4
  5. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    If it's years ago, they ain't known to be good. Check everything.
    #5
  6. boxerboy81

    boxerboy81 Stay Horizontal

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    Sounds like a carb refurb is in order. How were they prepared for the lay up? It might just need a clean out of petrol gums and sediment etc from the passages, esp around the needle & jet?
    #6
  7. ignatz72

    ignatz72 call me iggy

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    Then again it could also be remnants of old, flat, humid gas as Boxerboy mentions - in the tank too though.

    You might want to drain the tank to inspect for anything nasty. Did you use any kind of fuel stabilizer prior to storage?
    #7
  8. squish

    squish Out of the office.

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    Check the condition of your needles. The pit if they are stored where there is a high level of moisture in the air.
    #8
  9. patanga

    patanga BMWAirheadsDownunder

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    Plus one to 'boxerboy's' & 'Ignatz72's' comments. Chunky bits in contaminated fuel is the first thing I'd be looking for. If so, also remove & strip fuel taps & carbs, blow out ALL fuel lines and galleries, replace filters if in the hoses. Good luck.

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    #9
  10. Wirespokes

    Wirespokes Beemerholics Anonymous

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    How were the carbs cleaned? I suspect the atomizer area and o-rings.
    #10
  11. OZbeemer

    OZbeemer Been here awhile

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    What about the boots from the cylinder heads to the carbs leaking? Would this cause this type of problem?

    I need to replace them anyways, but was wondering if this could be the culprit.

    Bike was stored with stabilizer, thought that the carbs had been run dry, but found a bit of fuel in each float bowl.

    Wire-carb was disassembled and cleaned/compressed air and such.
    #11
  12. R100RT Mark

    R100RT Mark Been here awhile

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    The fluids in all the normal carb. cleaner sprays eats away at o-rings, and the diaphragms if you are careless enough to get any on them.

    Most "routine" overhauls do no disturb the throttle plate, so keeping carb cleaner away from its o-rings is important.

    Also, for a "quick" overhaul, many do not disturb the enricher circuit, and keeping carb cleaner away from its o-ring is also important.

    My 1994 R100RT with 35,000 miles developed a very slight surging when held at a "constant" 4,000 rpm range throttle which I cured by renewing the jet stock, idle jet and mixture screw o-rings. I did them all in one swoop, so cannot be definitive about which one was the problem, but one jet stock o-ring was very "gunky" and one mixture screw looked like it had been pinched at some time. I think the source of the problem was a lack or lapse of attention by me during an earlier clean/overhaul. Quick, easy and cheap fix.
    #12
  13. Wirespokes

    Wirespokes Beemerholics Anonymous

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    The rubber intake manifolds shouldn't be a problem at higher RPMs - more at low RPMs and idle. Check that the intake spigots are tight in the heads. You could have a number of things going on, not just one.

    Check the diaphrams closely - stretch them - sometimes that's the only way to see a hole.

    Sometimes varnish builds up in the passages and requires a soak in carb cleaner or an ultrasonic. I've heard of using fishing line poked through the orifaces to clean them out. Minimumly, spray carb cleaner through the small air channels and follow with compressed air.
    #13
  14. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    And one thing that is sometimes overlooked in the "disassemble and spray it" type of overhaul is the needle jet AND the atomizer with it's associated air and fuel passages. Goo and goop can build up around the atomizer, blocking the holes and upsetting the air:fuel mixture (shifting it towards the rich end).

    Typically a surge is thought to be a lean surge, but it can be caused by a too-rich mixture. The OP might want to go out and run the bike for several miles under surging conditions and stop and do a plug cut to see of the mixture is rich or lean.
    #14
  15. OZbeemer

    OZbeemer Been here awhile

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    Thats the plan. I just put fresh plugs in it so I can get a good read as to what it looks like. I am going to get the o-rings for the carbs as well as the intake boots.
    #15
  16. boxerboy81

    boxerboy81 Stay Horizontal

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    Easy job, 5 minutes...remove the main jets and ensure they're not obstructed. You might get lucky.
    #16