how do you check a magneto?

Discussion in 'Bike Mods, Performance, Maintenance & Repair' started by cjs, Feb 25, 2003.



  1. cjs

    cjs Rookie DRN Member

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    I am trying to rebuild an 81 rm125 and I have gotten as far as cracking the cases and puting in new seals and checking the bearings. I also installed different trans gears from another motor. I want to check to see if the magneto is still good. When I orginally pulled off the cover the rotor and the metal part of the magneto was rusty. I am assuming that water got in there to cause the rust at some point as the bike had been sitting for a long time. My friend told me to hook it up to a power souce to see if it could run a current through it and test it with a test light. It didn't pass that test but I don't know if that is the right way to test it. I do have the manual for this bike but I am a little mechanically challenged. So if someone could help me out that would be great. I am using this bike as an intro to 2-stroke repair as I will have to do alot of work to get it going by spring. Thanks


  2. jmics19067

    jmics19067 Lifetime Sponsor

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    Basicly speaking a single stator coil is one long wire. The best way to check it is with an ohm meter,using your manual. the idea is too check both ends of that wire for resistance, the resistance values pretty much tells you that the one piece of wire is one piece of wire in good shape. if resistance is too low that means there is a break in the insulation of the wire and it is short circuiting to itself. if the resistence is too high that mean there is a break in the copper of the wire. also check for resitance from one of the ends to ground if the resistance is infinite you know the wire isnt shorting to ground if there is any conductivity between the wire and ground it is no good.

    Your manual should be able to tell you what color wires should have what resistance. the idea of using an external power and test light is both good and bad. the idea is good for checking continuity but when I tried rewiring my lighting coil there was over 80ft of 22 gauge wire and depending on what you used for a light could easily smoke the wire. Also a lot of stator coils are grounded to the stator plate, In the case of my Yamaha I have 2 ignition coils 1 trigger coil and the stock lighting coil was all grounded meaning one end of each stator wire is pretty much hooked up to all the other one ends of the other coils. Doing what the manual states letter for letter with ohm resistence cannot be beat



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