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  1. #1
    CB750 New Member mustardCB750f's Avatar
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    1975 cb750f -purpose of the black wire.

    I bought this bike a little over a year ago in a box so I had no reference as to where anything went beforehand. The bike is almost complete now but I've been dealing with issue after issue on the wiring side of things, if seems like every time I fix something, something else stops working. I had the bike running on all 4 cylinders and getting it ready for a carb sync when I ran into the issue of blowing the main fuse immediately after the bike would start, I traced the issue to crossed wires on the regulator, that seems to have fixed the blowing fuses issue, but a new one arose... I was now only running on the 2 outer cylinders fed by the yellow lead off the points. As for the inner cylinders, the plugs are not fouled and if I switch the wires for the coils I get power to them. I seem to be also getting power through the black and white wire coming from the kill switch. I checked the rest of my wires and they all seem to be connected in the proper places, the only thing I can think of is when I had the wires switched on the regulator, I might have fried it as I am getting power throughout the black wire. If anyone can help me or give me some advice it would be greatly appreciated as I am now at a standstill.

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  2. #2
    CB750 New Member mustardCB750f's Avatar
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    Quote Originally Posted by mustardCB750f View Post
    I bought this bike a little over a year ago in a box so I had no reference as to where anything went beforehand. The bike is almost complete now but I've been dealing with issue after issue on the wiring side of things, if seems like every time I fix something, something else stops working. I had the bike running on all 4 cylinders and getting it ready for a carb sync when I ran into the issue of blowing the main fuse immediately after the bike would start, I traced the issue to crossed wires on the regulator, that seems to have fixed the blowing fuses issue, but a new one arose... I was now only running on the 2 outer cylinders fed by the yellow lead off the points. As for the inner cylinders, the plugs are not fouled and if I switch the wires for the coils I get power to them. I seem to be also getting power through the black and white wire coming from the kill switch. I checked the rest of my wires and they all seem to be connected in the proper places, the only thing I can think of is when I had the wires switched on the regulator, I might have fried it as I am getting power throughout the black wire. If anyone can help me or give me some advice it would be greatly appreciated as I am now at a standstill.

    Sent from my LM-Q710.FGN using Tapatalk
    I've figured it out! So I'm still not 100% sure about the purpose of the black wire, but I believe it's anauxiliary power source or a common ground. The issue was in the points system, the yellow wire going to coil for cylinders 2-3 was frayed and contacting the base plate, I repaired the connection and I had spark on all 4 cylinders. I even had the time tonight to sync my carbs.

    Sent from my LM-Q710.FGN using Tapatalk

  3. #3
    CB750 New Member mustardCB750f's Avatar
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    Quote Originally Posted by mustardCB750f View Post
    I've figured it out! So I'm still not 100% sure about the purpose of the black wire, but I believe it's anauxiliary power source or a common ground. The issue was in the points system, the yellow wire going to coil for cylinders 2-3 was frayed and contacting the base plate, I repaired the connection and I had spark on all 4 cylinders. I even had the time tonight to sync my carbs.

    Sent from my LM-Q710.FGN using Tapatalk
    Here is the picture of the repair

    Sent from my LM-Q710.FGN using Tapatalk

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