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  1. #1
    Join Date
    Aug 2022
    Location
    East Berlin, PA
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    5

    Default 1974 Chrysler 120HP Conversion to MSD Ignition

    I am struggling to make this conversion work. The original CDI system had a weak spark under load and would load up at idle and not perform under load. I decided to convert the whole thing to automotive ignition, replacing the CDI and coil with the following:
    1. Flame Thrower 1.5ohm coil
    2. MSD Street Fire 5520 multispark box
    3. Carbon core wires (required by MSD box)
    4. smaller gap plug. (currently using NGK BP8H-N-10 plugs), but I think they are the problem.

    Symptoms: Starts up with a weak idle. A little stumble, but seems to stay running for a minute or so. It has great performance at and above 2000 rpm. Below that it stumbles and gets progressively worse, until it won't idle and we paddle back to the boat ramp.

    It has a new power head so we have to run extra rich oil mixture (25:1), which is probably contributing to the problem.

    The cap looks good and the rotor to cap tolerance has been fine tuned to within .005. The current plugs are gapped at .030 and were originally at .035 with the same symptoms.

    Carbs are refreshed and clean and mixture screws have been fiddled with to achieve highest RPM at idle.

    I am looking for a small gap surface gap plug to try as I feel the traditional gap plugs are collecting raw fuel in the valley around the electrode and "loading up". Anyone have any success with an automotive ignition (with or without MSD) and want to recommend a plug to use?
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  2. #2
    Join Date
    Aug 2022
    Location
    East Berlin, PA
    Posts
    5

    Default Re: 1974 Chrysler 120HP Conversion to MSD Ignition

    Anyone?

  3. #3
    Join Date
    May 2015
    Posts
    2,852

    Default Re: 1974 Chrysler 120HP Conversion to MSD Ignition

    Quote Originally Posted by 32Tudor View Post
    Anyone?
    Go on YouTube there is a guy there with a great video how to do that.

  4. #4
    Join Date
    Aug 2022
    Location
    East Berlin, PA
    Posts
    5

    Default Re: 1974 Chrysler 120HP Conversion to MSD Ignition

    Quote Originally Posted by flyingscott View Post
    Go on YouTube there is a guy there with a great video how to do that.
    Thanks for the suggestion. I have watched every video I could find on this topic. I never found one that used an MSD box. In fact one of the comments on a conversion to automotive coil video, is where a guy named "MVEVERETT" said without an MSD box it will load up at idle and lose some RPM of the top end. I have reached out to MVEVERETT and several other people. I did get a response from one guy, but he did not have any additional info for me.
    If you have a particular video in mind, please post a link. Maybe I missed it somehow?

  5. #5
    Join Date
    May 2010
    Location
    Sebastian ,Fl
    Posts
    3,350

    Default Re: 1974 Chrysler 120HP Conversion to MSD Ignition

    Nordin, from Sweden might be able to help.
    Send him this post. He can't get this site where he is.
    He's pretty knowledgeable about changing the iginition's around.



    [email protected]
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  6. #6
    Join Date
    Aug 2022
    Location
    East Berlin, PA
    Posts
    5

    Default Re: 1974 Chrysler 120HP Conversion to MSD Ignition

    Quote Originally Posted by Jerryjerry05 View Post
    Nordin, from Sweden might be able to help.
    Send him this post. He can't get this site where he is.
    He's pretty knowledgeable about changing the iginition's around.



    [email protected]

    Thanks. I sent him an email.

  7. #7
    Join Date
    Sep 2021
    Location
    Fort Worth
    Posts
    1

    Default Re: 1974 Chrysler 120HP Conversion to MSD Ignition

    Hey 32Tudor. I have a very similar motor in which I have successfully converted using a 6v oil filled coil with internal resistor and automotive spark plugs. It completely eliminates the cdi box and has been very reliable for me...Maybe an option for you. Ive done a lot of research and decided against the msd on that motor for several reasons

  8. #8
    Join Date
    Aug 2022
    Location
    East Berlin, PA
    Posts
    5

    Default Re: 1974 Chrysler 120HP Conversion to MSD Ignition

    Update: Using a surface gap plug with relatively small gap (.033) has helped a lot. I now have a stable idle and top end works good. I still have a stumble in the mid range (2000-3500rpm). Carbs are tuned, total timing is at 32 degrees. Base idle timing is at 8 BTDC. I would like the hear the reasons you do not like MSD for this. So far I feel it is helping on the low RPM side and giving me a more complete burn at idle. Plugs are "NGK 2522 Standard Spark Plug - BUHX".

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