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Thread: Magneto speed ???

  1. #1
    Join Date
    Mar 2018
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    Default Magneto speed ???

    I am thinking of making up a magneto tester and not sure if the mags run at crankshaft rpm or 1/2 crankshaft speed. The 4 cylinder engines run much slower than the pony starting engines and I would like to be able to test both. I dont want to overspeed them. Old Croak Magnetos gave me a bunch of reasons to not run the mags without being fully connected to plugs. Any help will be appreciated. Thanks Grant.

  2. #2
    Join Date
    Jan 2009
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    Colorado Springs and Waitsburg WA
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    On a 4cycle engine it would have to run at camshaft speed I would think.

    Also the shop that worked on my magneto made it clear that there must be a place for the magneto output to go else it will eventually short the induction coil.
    John Liebermann
    2 9U's, 5J, IHC544, Ford860, (All working farm tractors)
    cojhl2@gmail.com

  3. #3
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    Nov 2006
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    Faunsdale, AL USA
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    4,418

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    Variable speed drill?

    If you have a jack shaft drive instead of direct drive you might want to use a couple step pullies like a drill press to get different speeds.
    D2-5J's, D6-9U's, D318 and D333 power units, 12E-99E grader, 922B & 944A wheel loaders, D330C generator set, DW20 water tanker and a bunch of Jersey cows to take care of in my spare time

  4. #4
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    Nov 2006
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    BAxter,Iowa
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    The only magnetos that I know of that run crankshaft speed are the early two cylinder John Deere's. For checking magnetos you need a slow speed for checking spark when the impulse is tripping and a higher speed for higher rpms. On my machine I have a 10 to 1 reduction to check the impulse. Most of my info says that spark should jump a 9mm gap at all speeds. A good one will do better than that.

  5. #5
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    Feb 2009
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    Shandon, CA.
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    Kal covered it very well. Some run at crankshaft speed and the 2:1 reduction is between the mainshaft and rotor drive, all you need be concerned about is to get the mag speed past impulse if it has one and have a place for the spark to find ground not much over .300, they will jump many times that amount.
    Last edited by Tad Wicks; 07-10-2020 at 08:48 AM.

  6. #6
    Join Date
    Mar 2018
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    After reading these responses and getting some coffee into my brain, for our gas cats, it has to be half speed. I think I will use a small variable DC motor and maybe a multi speed pulley setup like CCjersey said. I am more concerned with over speeding and possible damage if I dont have it close to working speed. Being able to run one at just about the speed of hand cranking an engine will also be needed. Engine mags must operate at something less than 600 rpm and pony mags at something like 1800 rpm. That is a lot of difference. I have a stack of mags that my Dad and brother and I have collected over the years and I would like to test them. Thank You All. Grant.

  7. #7
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    Nov 2006
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    West Jordan, Utah
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    Sorry most mags run at crankshaft speed unless it is a six cylinder mag and then they usually run at 1.5 X crankshaft speed. All gas Cats run at crankshaft speed. D2,D4 and early vertical starting engine mags run at crankshaft speed. JD 2 cylinders run at crankshaft speed. Case L, LA, C, DC, SC etc run at crankshaft speed. I could keep going.

  8. #8
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    Mar 2018
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    So you are saying that the mag fires twice for each power stroke? If it turns at crankshaft speed it passes each mag contact (in the cap) twice for each power stroke. That means it fires again on tdc of the exhaust stroke. Doesnt seem logical but maybe it is that way. Now I am really curious. Grant.

  9. #9
    Join Date
    Nov 2006
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    Pittsford NY
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    What Ken means is that the armature in the magneto runs at crank speed. In a wasted spark engine (fires every rotation of the crank but the spark at the overlap between exhaust and intake does nothing useful), and in two-cycle enginess, the distributor rotor also rotates at crank speed. In a non-wasted spark four-cycle, the distributor runs at half crank speed. There may be mags where the armature runs at a different speed to the crank so if someone's aware of one, we'd all be interested I'm sure : )
    Cheers,
    Neil.

  10. #10
    Join Date
    Dec 2006
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    victoria australia
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    Default Old Croak Magneto Manuals

    Hi Team,
    some magneto's run at engine speed with a 2:1 reduction at the distributor rotor, as said above, ie:- 2Ton, Twenty Two etc. so is standard for most gas Cats as said above , and some --usually single cylinder engines can run at half engine speed or can run at engine speed for 2 stroke engines or have the wasted spark at valve overlap in 4 strokes to give stronger starting speed spark, as said above.

    Our Dealer Electrical test bench could run some 3,000rpm, from memory, to enable testing of alternators, generators etc.

    Link below for some Eisemann magneto's of the day, courtesy of Old Croak Magneto, shows the two types of magneto discussed here along with some inside details of them showing the 2:1 reduction rotor distributor gearing.

    https://oldcroak.com/gv-4-gv-2h-gv-2q/

    https://oldcroak.com/eisemann/

    Great Project for you to build a test bench.
    Regards,
    Eddie B.

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