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Thread: Coil question

  1. #1
    CB750 Member Ajsharp991's Avatar
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    Coil question

    Hey everyone, coils on my bike are aftermarket and measure out to 1.3 ohms on the primary side. My question is will that cause me problems with the rest of my ignition system ? 1.3 seems low. Also what is stock coil resistance?

  2. #2
    CB750 Guru amc49's Avatar
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    OEM coils are 3 ohm.....................the first thing that goes is the spark units (ignitors), the switching transistor in them will not handle the extra amp passed by low coils. They are pretty touchy about it.

  3. #3
    CB750 Guru amc49's Avatar
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    ..............................double post
    Last edited by amc49; 11-30-2019 at 09:25 AM.

  4. #4
    CB750 Guru amc49's Avatar
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    I may be full of crap...............

    https://www.motorcycleproject.com/te...ion_coils.html

    That guy (Mike Nixon) was a Honda tech forever on DOHC and pretty much knows his stuff.

    What I passed down in the response above was 'what I've picked up or heard', I have had no experience with spark unit damage directly, mine has much more been along the lines of what is posted in the above link, and I agree with the last note #3 in it. As in, aftermarket coils generally for sale are not spit. I for one tried out Accel coils at lower ohm and quite simply was not impressed with them, I then came up with the GM HEI coil/module setup with .5 ohm coils and the difference was quite striking back in the very early '80s, THAT one would fire plug gaps over twice as wide as the high falutin' Accels. I know for sure that using coils THAT low WILL eat the spark units, it was reported a bunch of times in the DOHC websites when guys did partial mods to only use the GM coils because they could get them from junkyards for pennies.

    http://www.cb750c.com/modules.php?na...wtopic&t=17189

    YET, that info comes from the SAME guy who did the above link! So, what's it gonna be?

    Welcome to Confusion City..................fun, huh?

    I CAN tell you after lots of personal hands on with really big carb auto race engines that you can appear to have totally rejetted a carb simply by increasing spark voltage if it was badly needed, the carb could appear too rich and then suddenly the stronger spark makes it go lean. BTDT a hundred times, so that part is 100% true. Weak spark always makes the carb seem too rich, the intertwining there is hard linked. Why just really bumping spark up can clean idle quality up like you would not believe.
    Last edited by amc49; 11-30-2019 at 09:41 AM.

  5. #5
    CB750 Member Ajsharp991's Avatar
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    Interesting... I picked the bike up from a buddy a few years ago and just started work six months ago on it. Any way, he had purchased "OEM Style" coils from Vintage CB750. Now I am convinced that is what melted the spark units . Do you know of any coils I can purchase that will hold up and not wreak havoc other places in the system? Was thinking the dyna 3 ohm but I've read some bad stuff on here. Thanks for the reply. I appreciate any knowledge

  6. #6
    CB750 Guru amc49's Avatar
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    If you are talking about melting goo coming off bottom of the units they will do that during a hot summer even if you never run the bike for a minute. The potting compound was picked in error on the early ones. The units may well be fine, I drove on melted ones for years, the potting is simply tar and continues to coat the electronics even though much of it is gone.

  7. #7
    CB750 Member Ajsharp991's Avatar
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    Ok, I will have to do some further testing on the whole system after I finish this harness. Dont need to go throwing parts at it

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