Magneto coil keeps burning out

Discussion in 'Motorized Bicycle Trouble Shooting' started by THowell1975, Oct 24, 2016.

  1. THowell1975

    THowell1975 New Member

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    I have searched this forum and haven't found an answer for this one. I have a problem with my magneto. It keeps burning out. The wires coming out the side of the plastic and are soldered to the base burn up. I've tried to resolder them and they may work for a couple of miles and then burn up again right at the solder joint. I've added enough solder on top of that and tried again and it ran, but burnt up again and now there's no more wire left to resolder. I've burnt up three of these the same way, so I don't think the problem really is the magneto anymore. It's like I'm getting too much voltage for the wire or something. Could this be the CDI? Any way to check?

    I also bought one more magneto that doesn't have the white wire at all, just the blue and the black (and no pesky wires soldered to the base) and it worked for a few days and then it quit. It actually ohms out good between blue and black, but it won't make any spark anymore. I tried one of my "resoldered" ones after that one and it worked for a couple miles until that little wire burned up again and I couldn't resolder it anymore. I'm completely lost and out of magnetos and don't want to order any more until I figure out this issue. Any help would be appreciated. Thanks!

    Oh yeah, on the ones with the white wire, it is cut off and heat shrinked, so no way is that shorting anything out. I'm thinking that if it fires up and works, everything is good in the circuit. But if it burns out after just a few miles, there must be something else. Or are these cheap little magneto coils THAT cheap that I need to keep a half dozen spares at all times?? My spark plug is the factory one and I assume it's good. Every time I add a new coil, I get good spark again. Until the coil burns up...
     
  2. Agreen

    Agreen Member

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    Well, the input impedance on the cdi should (in theory) be pretty high. I would probably need to see a schematic before making that call. Anyway, check the resistance of the cdi, but it may not really tell you much. I'd try a new cdi and magneto.

    How is your kill switch wired?

    Can I also see a picture of how your solder job looks?

    And no, the magnetos don't burn up that fast normally.
     
  3. THowell1975

    THowell1975 New Member

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    Well, I don't have any pictures of my solder work, but I'll promise you it ain't very pretty. I've resoldered each one 2-3 times. They worked, so I assume I did a good enough job. The kill switch was taken out of the equation a long time ago, so that's not an issue.
    I forgot to mention, I did try a new CDI and spark plug already. Maybe i should try all three at once?
     
  4. Venice Motor Bikes

    Venice Motor Bikes Custom Builder / Dealer/Los Angeles

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    Yes, try all three at once & don't zip tie the blue & black wires too tight to the frame... just to be sure that doesn't cause a slight short. ;)
     
  5. crassius

    crassius Well-Known Member

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    have you pried up on that end of crank with a screwdriver? your main bearing may have gone bad and it is shaking the coil apart internally
     
  6. THowell1975

    THowell1975 New Member

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    Yes, the bearings seemed good and crank seems solid. It's a fairly new engine with less than a 300 miles on it.
     
  7. crassius

    crassius Well-Known Member

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    your problem seems more than can be solved without experienced eyes on the scene - last thing I can think of is exhaust pipe dumping a lot of heat right there thru a crack or hole
     
  8. Agreen

    Agreen Member

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  9. Seabass

    Seabass New Member

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    I think id check the magneto clearance first. It should be .010 or about a business card thick. I'm wondering if the cdi is not getting enough volts due to this and as a result it is drawing more to achieve saturation in the primary winding of the cdi. Remember the same current goes thru the ground as the positive. I think the soldered joint is just the weak link. Maybe you could install an inline fuse holder on the hot wire to the cdi and keep putting in bigger and bigger fuses till it stops blowing them. Id start with 1 amp.

    I don't know but I'm sure I'd figure it with it in front of me.

    Not to set màgnito clearance, rotate crank till magnet is pointed at màgnito. Slip business card between crank and màgnito. Loosen mounts to màgnito. Magnet will suck màgnito to business card. Tighten mounts.

    Well at least that's how its done on Briggs, Tecumseh, Robin, etc....

    Look at how my computer is spelling màgnito!!! I'm becoming my french canadianalter ego of myself...lol
     
  10. Seabass

    Seabass New Member

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    I think you should check the magnet to magnito clearance. It should be .010 or about 1 business card. The thinking is that the cdi primary winding is not achieving saturation due to low voltage and as a result is drawing more amperage . The melting of the solder is just the breaking of the weak spot. You could check this amperage idea buy installing an inline fuse on one of the cdi primary wires. I start with a 1 amp fuse and keep increasing it untill it stops blowing. You know the Chinese make crap so it would not surprise me if the gap was set way off. Hopefully it is adjustable. Been a few years since i had a China girl open. Good luck I'll be watching this thread.

    Wow this is interesting. I'm probably about a mile off. I understand the cdi is not a coil pack but at the same time there is capacitance to it. Just put your meter on farad's and go across the primary wires

    I'm way over thinking this one I bet..haha I think I need a beer to dumb it down
     
    #10 Seabass, Oct 29, 2016
    Last edited: Oct 29, 2016
  11. Slogger

    Slogger Member

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    I don't think the gap is adjustable, except to make sure the iron core is centered up on its four mount screws with clearance all around for the magnet.
    Mine doesn't hit but I found some filings in there that mean it does, maybe only at high rpm with the shaft's flexing/thrashing and causing the loss of the gap and a little scuffing.
    All I could think of was to make sure its mounted centered on the 4 screws and checked for clearance. I suppose the outer faces of the magnet could be emory clothed for a little more.
    I could take off the core, look for the scuffs and adjust it away from there I imagine.

    I ride at middle rpms, the gearing won't let it scream, ever. No trouble with the coil or CDIs, maybe high rpm is breaking your wire.

    This is an interesting problem. :-||
     
    #11 Slogger, Oct 30, 2016
    Last edited: Oct 30, 2016
  12. crassius

    crassius Well-Known Member

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    filings in there means either that at one time it was off center, or that your main bearing is bad on that side allowing the crank to hop at high revs
     
  13. Slogger

    Slogger Member

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    Pretty much. It doesn't seem to make a noise or hurt anything, so da heck witit.
     
  14. crassius

    crassius Well-Known Member

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    If I didn't see bad movement in main bearing, I might just take armature off and file it out for more clearance - constant tapping there could be shaking your coil until it shorts internally.
     

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