Birotor CDI ignition
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  1. #1
    Fellow Frogger! andrewj's Avatar
    Join Date
    Jan 2004

    Default Birotor CDI ignition

    Firstly, well done gibgib in getting the forum up and running again. In this "on demand" age, it is all to easy to forget how much effort is put into keeping it takes to actually make things happen.

    So I am now logged in under a new name, and forum is now in English, not French.

    Back to the car...

    I have an issue with the Birotor. After a hassle free trip up to Launceston for Citin, it is now coughing every now and then but refusing to start. There is plenty of fuel going through an I am pretty sure it is electrical.

    I have checked the obvious stuff- plugs, swapping leads etc, and am now pretty sure it is the CDI module or the coil.

    When I put a brand new plug straight off the ignition coil I get "thin" white spark. When I try to draw an arc to earth, I can get a good 5mm spark, but again with no "body" to it. So I am presuming that I am getting reasonable volts, but insufficient amps.

    Working on the assumption that faultly CDI unit would result in in-sufficient volts, the first step is to replace the coil. This is where I need some wisdom.

    The original coil is obselete and un-available, so the question is, how do I find a similar coil that will do the job?

    With CDI, the coil is essentially acting as a transformer. So going back to first principles, it would be necessary to match turns ratio, flux carrying capacity of the core, resistance of the primary and secondary coils and of course voltage withstand rating.

    The Bosch catalogue shows a unit one number different that was fitted to CDI 911's of the same era.

    Do I just bung it in and see what happens, or should I dig deeper?

    Your opinions please......


  2. #2
    Now go make me a sandwich Hotrodelectric's Avatar
    Join Date
    Nov 2010
    San Diego, California, USA


    Didn't the Mazda RX2 and 3 also have some form of CDI? The reason why I'm asking is I'm thinking that the rotary needed a little bit more to get it going- I'm definitely no expert on rotaries. Then again, the difference between the Porsche coil and your GS one could be as simple as the color of the paint applied.

    I guess the whole upshot is go ahead and try the Porsche coil. You certainly can't hurt anything with it. Just make sure that the internal/external resistor is either there, or that you need to buy one.
    The measure of your character isn't what you do when people are watching- it's what you do when they aren't watching.

  3. #3
    1000+ Posts Ken W's Avatar
    Join Date
    Sep 2002
    North Brisbane


    I built myself a CDI back in the 1970s when they were all the rage. They were basically a inverter power supply that charges a capacitor to about 400 volts. When its time to fire a sparkplug, this capacitor is shorted across the coil primary by triggering a thyristor. The coil acts as a transformer and will generate up to 40kV unless the spark plug breaks down at lower voltage.

    I just used an ordinary coil on my CDI system. Because there is a lot less primary current, there is a lot less heating in the coil but I guess the secondary insulation gets a bit more stress.


    Ken W

  4. #4
    1000+ Posts Haakon's Avatar
    Join Date
    May 2002


    I can help with NSU Ro80 parts. Did Citroen use a similar Bosch ignition system with the NSU motor as well?
    I tried to drown my sorrows in alcohol, but the bastards learnt how to swim

  5. #5
    1000+ Posts bluey504's Avatar
    Join Date
    Oct 2010
    Dandenong Vic


    Check the resistance of the coil against the other coil (I'm making the wild assumption that it is a twin coil setup as per Mazda.) Both the primary and secondary windings as early Mazda rotary coils failed due to the 'unique' field collapse requirements. This happened to a greater extent when standard coils where used as replacements ie Bosch GT40 and GT40R as they where 25% of the price of a factory coil!
    If you can explain how the system is set up I'm pretty sure I can help. Even stupid stuff like the code off the CDI cover helps as even the Big Boy's shop at Tandy!
    If all else goes 'pear shaped' then it is pretty east to convert to an electronic system with Ford/Holden bits that will deliver a spark just shy of a lightning strike!

  6. #6
    Member ss2115's Avatar
    Join Date
    Nov 2010


    You can use almost any standard coil with CDI.
    Don't employ a ballast resistor - the beauty of a CDI is that it will still fire a great spark right down to a coil voltage of some 7 - 9 volts.

    Don't use a sports coil such as a GT type coil or any version of - use a standard everday coil.

    The only problem you might have is that if its a cheaply constructed coil, the spark might breakover the insulation giving you a poor spark at the plug (if at all).

    *And this might possibly be whats happening anyway as coils can breakdown over time with hailine cracks, heat, age etc.

    Try any standard coil you want to.

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