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  1. #1
    CB750 Member forgiven's Avatar
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    1973 cb750 ignition

    I put a K6 motor in a K3 frame (and K3 wire harness). The ignition switch is a three position switch (off, start, start with lights). I replaced the old point ignition system with an electronic ignition and used power from the brakes wire to power the electronic ignition. It runs well in the second position (start). Unfortunately, when I switch to start with lights I loose ignition power and the engine shuts down but the lights work (except the brake light). I've been all through this bike and this is confounding. Any insights anyone may have that would be helpful would be greatly appreciated.

  2. #2
    Moderator dirtdigger's Avatar
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    Why would you not use the power that goes to the ignition coils?

  3. #3
    CB750 Guru amc49's Avatar
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    X2, the brake wiring is likely too small to carry the extra amp needed there. Why the OEM setup is standalone.

  4. #4
    CB750 Member forgiven's Avatar
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    Quote Originally Posted by dirtdigger View Post
    Why would you not use the power that goes to the ignition coils?
    Proximity. Based on these comments I'll take power from the coil supply. I hadn't considered inadequate power supply.

    Thanks for the comments.

  5. #5
    CB750 Member forgiven's Avatar
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    Ignition

    Quote Originally Posted by forgiven View Post
    Proximity. Based on these comments I'll take power from the coil supply. I hadn't considered inadequate power supply.

    Thanks for the comments.
    It looks like the power supply stops when I put the ignition switch in the third position (go with lights) using a tester. I then tested power at the ignition switch. The black wire (the same one used at the brake switch I connected the electronic ignition to) has power in the second position (on) but looses the supply in the third position (on with lights). Based on this would I be correct in concluding the ignition switch needs to be replaced?

    I don't mind wrenching but electrical issues toss me for a loop. Thanks for any insights anyone may have on this.

  6. #6
    Moderator dirtdigger's Avatar
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    Click image for larger version. 

Name:	20190705_095405(1).jpg 
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ID:	9040 Your switch operation is normal.

  7. #7
    Moderator dirtdigger's Avatar
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    Sorry for the sideways pic

  8. #8
    CB750 Member forgiven's Avatar
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    Thanks dirtdigger. It must be a difference in the wire harnesses between the two years. In the on position I don't get lights, in the park position I get lights, no ignition. I suppose I could bypass the ignition switch with a kill switch to the electronic ignition wiring. What a pisser.

  9. #9
    Moderator dirtdigger's Avatar
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    If you just wire it to the ignition coil power it will work as intended, no need to add in another switch.

  10. #10
    Moderator dirtdigger's Avatar
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    In the on position you still need to turn the headlights on with the light switch

  11. #11
    CB750 Member forgiven's Avatar
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    Digger, your logic is unassailable. I embarrassingly admit I cannot locate a power supply to the coils. I see where the ignition wiring comes into each coil and where it goes out to the plugs. There is one wire common between the coils (black/white) however, it is not a power supply (I believe it to be a ground). I don't believe the wiring feeding the coils from the electronic ignition to be a constant power supply. I'm going to have to remove the coils to see if I can ID a supply other than the wiring from the ignition. It is very possible I don't understand power to the coils but it seems to me a pulse from the ignition is run to the coils, gets jacked up in voltage then sent to the plugs all in time. I appreciate all your comments digger. The reason I considered a kill switch is because I could pull a power supply from the battery realizing I would still need to use the switch to run the lights. It was because I cannot seem to find a constant power supply at the coils to tap as you suggested.

  12. #12
    CB750 Guru amc49's Avatar
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    The black/white wire IS power to coils, it is the yellow or blue off each one that is the ground through points on each on the original setup. Constant power runs through coils to the breaker points to only be pulsed or chopped up in pieces then.

  13. #13
    CB750 Member forgiven's Avatar
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    amc49, thanks for clearing that up. My skill with the tester is suspect. I still have the issue of no lights when I have power to the ignition. The power supply to the coil is constant in the first ignition switch position (where I have no lights). When I hit the second switch position I lose power to the ignition but have lights.

  14. #14
    CB750 Member forgiven's Avatar
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    It's apparent that I didn't get the wiring right when I put the motor in the frame with different wire harnesses as the switch isn't behaving as the documentation digger provided indicated it should (position 2 and 3 are reversed). I put a K6 motor in a K3 frame. The wiring diagrams are the same K1-5 but changed with K6. It's funny but the brake light works with the ignition (regardless of where I wire the ignition from) but not with the lights. It's a bummer, the engine runs great but I'm limited to daytime driving.

  15. #15
    CB750 Member forgiven's Avatar
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    Amazing... change one connection in the wiring harness and everything is as it should be including the brake light. I appreciate both of you digger and amc49 helping me think through this. On to the starter issue.

  16. #16
    CB750 Guru amc49's Avatar
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    Assuming you have found out that the last key position is brake light on only and not headlight. The so-called 'park' position. Keeps you from getting run down from the rear in the dark but draws less power.

  17. #17
    CB750 Member forgiven's Avatar
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    Interesting enough, thanks.

    Changing subjects to the starter. The K6 starter switch (which I moved with the motor) has five leads three of which are from the starter button (the other two to the engine kill switch). The k3 wire harness (on the frame I moved the k6 motor to) has two leads on the starter relay. I'm trouble shooting the starter not working. If I separately power up Starter Relay then jump the other lead to the power completing the circuit shouldn't the starter turn? Isn't this similar to jumping the posts on the starter relay? I'm trying to determine if the starter relay is the problem.

  18. #18
    Moderator dirtdigger's Avatar
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    The engines are the same, leave the wireharness that was on the bike. No need to keep swapping stuff with the motor, use the components meant for the wiring harness that was on the bike you are using. Seems you are getting caught up creating issues by swapping parts. I would put everything that was on the K3(minus the engine) and it will probably all work as intended. If not at least you will be starting from stock and can troubleshoot from there.

  19. #19
    CB750 Addict Medyo Bastos's Avatar
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    Yeah, I agree with Dana. Donít throw too many variables into your equation.


    Sent from my iPhone using Tapatalk

  20. #20
    CB750 Guru amc49's Avatar
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    If you want...........K6 starter solenoid............

    Two big connections for battery to and out of solenoid for main starter amp.
    Small yellow/red is power in for the activating coil and small green/red is ground out for the coil.

    Small red/white is battery power or charging output from regulator. That one connects internally to one of the big cables, test with VOM to see which one and whichever one connects to that red/white is the positive big cable connector to BATTERY, the other big one will be solenoid to STARTER.
    Last edited by amc49; 07-08-2019 at 05:01 AM.

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