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Splitdorf magneto questions

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  • 02 Aug 2022 1:39 AM
    Message # 12870232

    Hey all. My 29 101 has a Splitdorf magneto and the spark has gotten very weak. Cold starting has gotten very hard and hot starts are near impossible.  Are there any parts that commonly wear out or go out of adjustment? I have the manual but thought I'd throw it out there to see if your collective wisdom could point me the right direction.


    Andrew Newstead

    Last modified: 03 Aug 2022 12:42 AM | Andrew Newstead
  • 02 Aug 2022 1:41 PM
    Reply # 12870816 on 12870232

    Hi Andrew,

    I had Marks Magneto in CT re-wrap the coil in mine.  He also "re-energized" the magnet.  Not really sure what that means but this thing practically spits fire now!  Might be worth giving them a call. 

    I can't upload a video so here is a link to a bench test.


    Last modified: 02 Aug 2022 1:42 PM | Daniel Quinn
  • 03 Aug 2022 12:44 AM
    Reply # 12871417 on 12870232

    Thanks for the heads up. I'd prefer not to have to send the parts away if there is something that I can do by myself but if no luck by winter then that's what will end up happening.

  • 03 Aug 2022 5:32 AM
    Reply # 12871544 on 12870232

    Hi Andrew,

    For info without getting into too much detail - a magneto has an armature with 2 coil windings - one with heavy gauge wire, and one with fine wire. Thick winding is low voltage and the fine wire is high voltage and connected to the plugs. 

    Inside the body of the armature is a capacitor/condenser which assists with creating a "fat" spark.

    The armature is connected to the points and to the spark plugs through a rotating connection called the slip ring.

    The main body of the magneto houses a permanent magnet - the energy from this is what generates the spark.

    The permanent magnet fits on top of a shaped pole piece which concentrates the magnetic flux at the appropriate point for the firing point of the magneto.

    In the case of V twins like ours, the pole piece is specially shaped to achieve spark at 42 degrees. But there is a compromise as the flux "density" will always be better for one spark plug than the other. Hence a good working magneto with points set to full retard will often only spark weakly or not at all on one plug until the points are moved back towards full advance.

    NOW ... Considering the above -

    With the magneto on the bike - Check the slip ring - these get contaminated with carbon from the brushes and can form a continual track thus bypassing the plugs. Also check the brushes.

    Check, and carefully clean the points.

    If there's no spark after this, then it would be worth removing the magneto and checking the turning resistance when rotated. There should be definite points of resistance where the points open to generate the spark. If there is little or no perceptible resistance, then it's likely the permanent magnet requires re-magnetising - so you'll need to take it to the repairers anyway.

    The original capacitor/condenser fitted into these old magnetos rarely fail unless someone has fitted a newer inferior part - this requires a rebuild to replace.

    Finally - the fine wire on the armature was traditionally hand wound and then taped over the outside with cloth and lacquer. The coil windings are basically loose wound and are susceptible to break. Therefore when the bike (and magneto) are hot, the windings expand and the break cannot be bridged by the electrical current. When it has cooled down, everything contracts and it runs for a while then stops again when hot.

    New rebuilds often include resin vacuum impregnation of the rebuilt armature to stop this happening.

    Hope that gives you some things to consider.

    Good luck with it.

    Regards Mick

  • 03 Aug 2022 7:16 AM
    Reply # 12871625 on 12870232
    Tim Raindle (Administrator)

    First up, check the magneto cap for carbon tracking, assuming you have an NS2 ? Many repro carbon brushes are slightly softer than the originals, and leave a carbon trail around the inside of the cap, which after a while will build up, and can cause a weak or faulty spark, as the current will transfer between the brass pick ups. A quick wipe with a cloth dampened with carb cleaner or similar should remove this, or failing that, a very fine abrasive like the 3m ultafine pads. You don't want to use anything to abrasive as this will leave a rough surface and exasperate the issue. Always try and replace the carbon brush the same way, as it will build up a polished profile to suit the cap. 

    It is also possible that the condensor is failing, the originals are constructed from thin sheets of mica and tin built up in layers, and will fail with age. The usual warning signs of this are failure when hot tho. The mag may cut out, and be difficult to start, you will scratch your head, and after a while ( usually just about when the breakdown truck arrives ) the engine will have cooled down enough and will start up first kick. A modern condensor can be fitted in the originals place.

    It should be possible to remagnetise the mag if you can find a local mag guy that has the equipment, as Daniel says, you can usually feel the pull of the magnets when you spin the mag by hand. A weak magnet will cause the symptoms you are describing. 

  • 03 Aug 2022 7:26 AM
    Reply # 12871630 on 12870232
    Tim Raindle (Administrator)

    On rereading, hot starts are near impossible. Check your condensor. A modern one can be hooked up on the outside of the mag to try it. you have to connect it to the lve feed to the points, and then thru a ground, you can rig this up with the condensor wired at one end to the feed, and then sandwiched thru the cap and earthed outside the mag . One guy on the 2012 Cannonball ran the whole trip with his mag wired like this.

    As an aside, the old timers used to refer to "watering the mag", ie cooling it down with a stream of liquid. Tried it once with a failing mag on a hot day and it worked. ( Mick, did this on my Sunbeam not far from where you are, on the way back to Canterbury from a West Sussex meet with Big Frank. Slip rings, by the way, think you are thinking about Lucas mags, not splitdorfs.) 

  • 03 Aug 2022 7:41 AM
    Reply # 12871645 on 12870232
    Tim Raindle (Administrator)

    A few years ago there were a batch of condensors on the market that failed completely aftr a few hundred miles.

    Marks Magneto in Connecticut will sell an appropriate modern condensor for a few dollars that is tried and tested.

  • 04 Aug 2022 1:26 AM
    Reply # 12872594 on 12870232

    Thanks to everyone that posted replies. I will re-read them a couple more times and then start the trouble shooting. I will keep you posted as there is still a couple more months of the year riding season and I'd like to get it out a few more times if I can fix the issue. All else failing I will send it away in the fall I guess.

    Thanks again. 


  • 04 Aug 2022 9:31 PM
    Reply # 12873968 on 12870232

    Def check  points are  clean and pit free  along with gap  ,  Condensers do fail  ,Usually open circuit  .  so  adding a 0.15mf   on the outside terminal to  Chassis    is a good test (common car  ignition condensers are  usually 0.2 MF  and will work    but  starting is slightly effected !  The Spltdorf is a Rotating Magnet  Magneto   The coil is stationary   within the magnetic field   Coils Fail and as you are  aware  most likely  96 years old  .  If Basic checks  hold no clues  then    its time take to  someone who knows and understands  them......There are  Splitdorf NS2   Information pages   on the Internet well worth  looking at .

      If you have a magneto  properly rebuilt   you will never look back.

      I gather you have changed spark plugs  ,  are they sooty    ?  that does not help with modern  fuels .    

      Have you checked your inlet manifold for  air leaks?   spraying  Quick start  or even WD-40  around your inlet nuts and nipples  will soon  show air leaks,as motor will stumble.

     regards Phil

  • 13 Aug 2022 1:10 AM
    Reply # 12882345 on 12870232


    Thanks for the advice. The points were quite dirty and the gap was 20 thou .  Need to get some new plugs as well as check my plug connections at the mag as well because there is intermittent missing on rear cylinder. Did some roadside repairs but now that it is back in the garage I can some better maintenance.  Should have more to report later this weekend.

    I actually think the magneto may still be ok but just in need of some tweaks due to zero use in the last 20 years.


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