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    voltage to coil measurements--too low?

    I've been battling rich conditions so I thought I'd check if weak spark was in the picture--coil resistances tested ok on both, but after reading another thread here I checked the voltage to the coils, here's what I got:

    12.3V at battery (resting since yesterday)
    10.2 at positive lead to coils at rest (orange/white wire connection to neg. battery terminal)
    8.3 at positive lead to coils while cranking

    Is this drop in voltage enough to be a potential cause for weak spark and rich plug readings? If so, any suggestions beyond weak connection somewhere else in the circuit? The connection joining the coils to the positive wire is new, and I'm getting the same reading at the coils themselves.

    Many thanks! Learning every day but electrical was never my strong suit...
    1978 GS 750 E
    1979 XS11 Standard
    1973 CB450

    #2
    Cause of week spark, yes. (And let me quess, the engine doesn’t start until let off the starter button).

    cuase of “rich symptoms”, I don’t know, I don’t have that sort of experience to say one way or other.

    as a test you could run a temporary jumper wire of main power to the coils. (Which is somewhat what the “coil relay mod” would do for you, but only when key is on).
    Last edited by Redman; 09-16-2021, 08:13 PM.

    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.... update 2 mocykl


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      #3
      I had almost the same voltages at the coil.
      I cleaned the fusebox, the ignition switch, the kill switch and the sneaky one was the coil supply plugs.
      That got me back to about half a volt below battery and it fired up first push of the button.
      90% of all carburettor problems are electrical
      97 R1100R
      Previous
      80 GS850G, 79 Z400B, 85 R100RT, 80 Z650D, 76 CB200

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        #4
        Brendan hit on a major point, there. There are MANY connections between the battery and the coils, you need to check/clean them ALL.

        Also remember that anywhere you see a "connector", there are actually THREE interface points, each of which can be corroded and cause problems:
        1 where the wire meets the brass terminal
        2 where the brass terminal meets another brass terminal (the other side of the "connector")
        3 where the brass terminal meets the next wire

        I'll probably miss a connection, but here is where you can start:
        - main + wire at battery
        - connector between battery and fuse box
        - junction of wire to input of main fuse
        - junction of output of main fuse to wire
        - connector between fuse box and main harness
        - connector near ignition key
        - contacts inside ignition switch
        - connector near ignition key
        - connector to fuse box
        - junction of wire to ignition fuse clip
        - junction of fuse clip to fuse input
        - junction of fuse output to fuse clip
        - junction of fuse clip to wire
        - connector from fuse box to wire leading to connector under tank
        - connector under tank to right handlebar sub-harness
        - connection to kill switch
        - connector from sub-harness to coil assembly
        - finally, the terminal on the coil

        Remember, at each 'connector', there are actually three interface points.
        Considering all these interface points, Brendan's half-volt loss is darn-near perfect.

        Comment


          #5
          Being the '78 750 does not have a fuse box and only one fuse, cleaning up connections isn't half as bad as it sounds...
          -Mal

          "The only reason for time is so that everything doesn't happen at once." - B. Banzai
          ___________

          78 GS750E

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            #6
            Thanks everyone. I haven't experienced the symptom of starting only when the starter is released (that I've noticed). The bike starts and runs pretty readily, though needs choke for a bit longer than it should.

            I'll do some more cleaning / restoring connections and repost new results.
            1978 GS 750 E
            1979 XS11 Standard
            1973 CB450

            Comment


              #7
              Welcome to the latest game, called "The hunt for missing volts".
              You're not just missing 2.3 volts, you have a much higher current in the system than it was designed for.
              Which can cause malfunctions and even kill electrical components.

              10 volt is really low, for me that is reason to start cleaning.
              8.3 volt sets off alarm bells for me.

              The reason i start from 'endpoints' is i find it easier to trace back towards the battery.
              And if the 'endpoint' is ok, i know the whole subcircuit is ok.

              The coils and the starter relais are powered by the RH handlebar switch.
              So if you have low power on the coils you probably also have that on the green/yellow
              wire that operates the starter relais.

              Next step would be to check out the RH handlebar switch.
              Those corrode inside and usually require cleaning.

              Be careful - there's springs in there that like to jump out.
              Rijk

              Top 10 Newbie Mistakes thread

              CV Carb rebuild tutorial
              VM Carb rebuild tutorial
              Bikecliff's website
              The Stator Papers

              "The thing about freedom - it's never free"

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                #8
                12.3V at battery (resting since yesterday)
                Your battery is getting old. Try any decent car battery and see if that helps or Unplugging (or turning off) your headlamp will also make more current available...

                Turning a four-cylinder bike over might be a bit of a strain for tan old battery but if it's spinning well...well it's always worth looking at the ignition electrical connections. Including the kill switch .Try testing voltage "across" the switch's connections when on ("voltage drop") It should be 0! Any good connection will test 0.

                I don't know what bike this is but if it has an ignitor,("TCI") this will drop voltage at the coils a lttle too.
                Last edited by Gorminrider; 09-17-2021, 10:32 AM.

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                  #9
                  Originally posted by allojohn View Post
                  Being the '78 750 does not have a fuse box and only one fuse, cleaning up connections isn't half as bad as it sounds...
                  Maybe not, but the principle is the same, ... check EVERYTHING.



                  Originally posted by Gorminrider View Post

                  I don't know what bike this is but if it has an ignitor,("TCI") this will drop voltage at the coils a lttle too.
                  His signature says "1978 GS 750 E".
                  If you're not living on the edge, you're taking up too much space.

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                    #10
                    Just an aside - you can keep the engine running in the presence of a weak spark by throttling it more than normal, that leads to rich running.
                    Not saying it's your running problem, but the voltage on cranking over is far too low, as is anything under 12V at the coils when running.
                    ---- Dave
                    79 GS850N - Might be a trike soon.
                    80 GS850T Single HIF38 S.U. SH775, Tow bar, Pantera II. Gnarly workhorse & daily driver.
                    79 XS650SE - Pragmatic Ratter - goes better than a manky old twin should.
                    92 XJ900F - Fairly Stock, for now.

                    Only a dog knows why a motorcyclist sticks his head out of a car window

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                      #11
                      Originally posted by Who Dat? View Post
                      .......
                      His signature says "1978 GS 750 E".
                      Ah yes, that reminds me: Weirdly, I can't see his signature when logged in. I can't see any one's signature unless I'm NOT logged in.

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                        #12
                        Originally posted by Gorminrider View Post
                        Ah yes, that reminds me: Weirdly, I can't see his signature when logged in. I can't see any one's signature unless I'm NOT logged in.
                        At the top of the page, click on "Forum", then go toward the center, click on "Forum Actions", then "General Settings". Scroll down to the "Thread Display Options" section, verify your settings there.

                        While you are right there, make sure the next two items are adjusted correctly, too. Set the "Thread Display Mode" to "Linear - Oldest First", then set the "Number of Posts to Show Per Page" to the maximum of "40 per page". That makes it easier to read longer threads without all the page-flipping.

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                          #13
                          ahaha Yes.. thanks. I had " signatures visible" unchecked. Some signatures here are awfully long and big and in a peevish mood, I was annoyed scrolling through them.

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                            #14
                            Thanks all, incredibly helpful. I'll start digging in this afternoon. Time to bust out the glasses for the wiring diagram...
                            Originally posted by Gorminrider View Post
                            Try testing voltage "across" the switch's connections when on ("voltage drop") It should be 0! Any good connection will test 0.
                            Didn't know this. So in the case of the kill switch, my touch points would be would be where orange and yellow/green connect to the switch? Test with key and switch both on?
                            1978 GS 750 E
                            1979 XS11 Standard
                            1973 CB450

                            Comment


                              #15
                              To measure voltage drop between two points of a circuit, the circuit must not only be “on”, it must have current actually flowing through it. So your meter actually reads the voltage that the circuit between the probes CAN’T handle.
                              Jim, in Central New York State.

                              1980 GS750E (bought used June,1983)
                              1968 CB350 Super Sport (bought new Oct,1968)
                              1962 CA77 305 Dream (bought used Feb,1963)

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