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Thread: GS850G '79 Done the carbs Weak spark now!!!

  1. #1
    Join Date
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    Default GS850G '79 Done the carbs Weak spark now!!!

    Hi to all the GS whisperers across the pond.
    I got my first GS850G and love it but it keeps throwing up problems. Latest one: Weak spark.

    To get you up to speed. I need a bit of knowledge thrown my way.
    Bought it as a runner with a cold 2 pipe. I stripped the carbs, gave 'em a sonic bath, re-shimmed the valves then found the airbox was a rusty piece of... everything. Pods, i thought! Got my Sigma jet kit, heaved on the pods and after only 5 or 6 attempts on and off the bench, got the old bird running on the kick starter ('cos my battery was flat after 3 months of me being away.) Revved up lovely in the garage in neutral and a solid tick over but only 200m down the road it konked out. Kick, kick, kick....Nada. Walk of shame back to the garage where i have just found i have a really weak spark.(And not just in my brain). This is a new world of hurt. I maaaaaaay have left the ignition on for a minute or two which i know is coil cooking madness and when i went to investigate, the right one was warm, the left, cold. I've got this far on sheer enthusiasm but this is stumping me 'cos well.. it's all old and i'm a novice mechanic, even though i am also old.
    I'm in Cornwall just reaching out to find a GS whisperer to help me over this hurdle..
    Anyone feel like they can throw me a bone here?
    Cheers now.

  2. #2
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    Itís a step by step process with these old critters.
    Well, you need a sorta charged battery and a functioning charging system to keep your ignition firing or youíll only make short excursions.
    1981 gs650L

    "We are all born ignorant, but you have to work hard to stay stupid" Ben Franklin

  3. #3
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    Wexford, Ireland
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    With a fully charged battery check the supply voltage at the coil plugs. It should ideally be within half a volt of the battery. Check them both for corrosion at the plug.
    Mine got a lot better when I cleaned the fusebox, the fusebox plug, the kill switch and ignition switch contacts.
    80 GS850GT
    79 Z400B

  4. #4
    Join Date
    Mar 2005
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    West Slope, OR
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    Does your bike still have points and condenser ignition?

    As above, check the voltage and the charging
    1978 GS 1000 (since new)
    1979 GS 1000 (The Fridge, superbike replica project)
    1978 GS 1000 (parts)
    1981 GS 850 (anyone want a project?)
    1981 GPZ 550 (backroad screamer)
    1970 450 Mk IIID (THUMP!)
    2007 DRz 400S
    1999 ATK 490ES
    1994 DR 350SES

  5. #5
    Redman's Avatar
    Redman is offline Forum LongTimer Past Site Supporter
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    Check battery voltage at 4k rpm. And check voltage at the coils (org/grn wire).

    Do you know that the sparkplug caps can go bad? can be removed from wire? can be checked with a meter?

    .

    Had 850G for 14 years. Now have GK since 2005.
    GK at IndyMotoGP Suzuki Display... ... GK on GSResources Page ... ... Euro Trash Ego Machine .. ..3 mo'cykls.



  6. #6
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    Do you have a workshop manual if not there is one Here. And a wiring diagram here . Another post of great information Here
    you need to start with a fully charged and good battery do the quick test Here
    Do you have a multimeter you are going to need one?
    the 79 model should have ignition points unless a po has converted it, the manual will help you to set/rest the points up.
    as all ready stated check the voltage at the coils (engine not running)
    if you have enough length on the ht leads unscrew the caps and snip 1/4" off the ends and screw the caps back on this will put the screw in the cap into fresh clean copper, if you can check the resistance of the caps should be around 5k ohms I think, report any findings on here and someone will reply.
    The big guy up there rides a Suzuki (this I know)
    1981 gs850gx

    1999 RF900
    past bikes. RF900
    TL1000s
    Hayabusa
    Francis Barnett

  7. #7
    chuck hahn's Avatar
    chuck hahn is offline Forum LongTimer Past Site Supporter
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    Unscrew the brass deals inside the boots that grip the plugs too.... They are slotted as you'll see. Dump out those dammmm resistors and replace them with a piece of thick copper wire shaved down to the same lengths. Note the 2 and 3 boots have springs and 1 and 4 don't. Put springs in then the resistor replacer and reassemble. No realistic need for them.
    MY BIKES..1977 GS 750 B, 1978 GS 1000 C (X2)
    1978 GS 1000 E, 1979 GS 1000 S, 1973 Yamaha TX 750, 1977 Kawasaki KZ 650B1, 1975 Honda GL1000 Goldwing, 1983 CB 650SC Nighthawk, 1972 Honda CB 350K4, 74 Honda CB550

    NEVER SNEAK UP ON A SLEEPING DOG..NOT EVEN YOUR OWN.


    I would rather trust my bike to a "QUACK" that KNOWS how to fix it rather than a book worm that THINKS HE KNOWS how to fix it.

  8. #8
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    Jun 2020
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    Thanks for the reply Chuck. I did the points and capacitors earlier. Nice spark. I checked both the coils with ohms and found that I had in fact cooked them both! So, always switch off the ignition when you're fiddling is the lesson here. Every day is a school day.

  9. #9
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    Having the support of you Stateside GS admirers is a big win for me. not so much love in the UK for these lovely lumps. I did a quick ohm test on the coils and got a big fail on the main wind which supplies the beef for the spark right? So my lack of attention to the ignition position has cost me time and a little more cash. Lesson learned.
    My next question: To chop or not to chop the Motard 4 into 1 pipe. Reducing the baffle by say 50% should gice more outflow, which is good right, since i've just jetted it. Any opinions will be received with thanks. Cheers now.

  10. #10
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    Jun 2020
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    Cornwall, UK
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    Nice tip on the leads sir. Chuck mentioned to get rid of the resistor in the caps too, which sounds sensible. I got a Clymer manual and just found a GS850GN specific Haynes manual on ebay. I'm sure now from a bench Ohm test that I managed to cook BOTH coils. Using the wire to HT test @ 2K settings I got a big nothing so that's my drama exposed i guess. Cheers now.

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