Multimeter to test magneto/coil

Discussion in 'Electrical' started by Fletch, Feb 10, 2011.

  1. Fletch

    Fletch Member

    I've done some searching and I'm a complete novice when it comes to anything electrical. I bought a multimeter but have never used one before (watched some youtube instructional videos).

    My engine has been dying from what I thought was fuel starvation going up long monster hills. In the past after it dies I can start it up again. This last time nothing.

    I'm not getting spark. Replaced plug, bypassed kill switch, tried new cdi, visually examined coil and magneto (no stray wires and not wet). So I'm to the point where I think it is probably the magneto from what I have read. There's a ton of threads on this, but I need the "for dummies" version!

    To test the magneto or coil, what do I need the multimeter set on, what wires do I connect to the positive and negative diodes and where specifically? I read about checking resistance and it says" blue magneto to black magneto should read 200 ohms +/- 10%." I feel like an idiot asking this, but is that with the magneto turning, and is that with the diodes touching the blue and black wires on the outside of the engine case, or inside the case?

    I really hate not knowing anything about electrical, and could use very specific instructions to make sure the magneto/coil is good/bad.


  2. shell shock

    shell shock Member

    hey, dont feel like an idiot. whether the engine is running or not should not really effect resistance. before you change anymore parts, scrap all the factory wiring. it is c**p. get a good automotive cable for your CDI. also, take your magneto to someone who knows how to solder, might just be a garbage solder job. it "COULD" also be the whole magneto assembly.. including the magnet. im not sure if this post will be of any assistance, but i hope it helps set you in the right direction

  3. Fletch

    Fletch Member

    I fixed it. It was the megneto. I replaced it with one from an old engine I have and immediate spark. Back up and running (for now).
    Last edited: Feb 10, 2011
  4. shell shock

    shell shock Member

    alas, good to hear! pesky magneto!
  5. Fletch

    Fletch Member

    Yep! Haha I had my fingers crossed man. I cut the white wire off completely from the magneto because I have no use for it and it is just a potential problem if it touches anything metal. I rewired the blue wire with my terrible soldering skills. It's ugly but it works!

    I have a question though. I don't know if you know the answer, but how exactly does a magneto go bad? It wasn't the magnet but how does a magnet go bad? I'm assuming it was the coil, but I don't know how the thing fails. Does it get overloaded? If your plug gap is too high does it have to "work harder" to get the spark across the gap? Can the coil and magnet be okay, but the metal part go bad somehow?

    I guess that's more than one question ;) Someone asked me how they go bad and I had no idea what to tell them.
  6. jaguar

    jaguar Well-Known Member

    Sometimes people replace the magneto coil, thinking it was bad, but all the while it was just a bad ground connection. The new one works for a while and then "goes bad" also because the owner is not cleaning the metal that is used for a ground path.
    But they can truly go bad by the very thin wire they are made of just burning thru where it was already too thin (maybe it was nicked, etc). They aren't high quality.
    A magnet can lose magnetic force by being dropped.
  7. Fletch

    Fletch Member

    That's good to know, thanks. The area my ground wire was wrapped around was definitely not clean. Although I tested for spark with another cdi as well with only the blue and black (ground) wires attached. I'm assuming you mean the ground wire that you run between the blue wires and not the one screwed into the magneto.

    Nice site BTW. Lots of good information. I have an 18mm carb coming tomorrow and just read your bit about not going to large on the carb. It comes with a 90 main jet, and I ordered an 85 and 80. Another member said he had luck on a 49cc HT with an 80 main. In another thread it looked like he had jumped up to 85. Are the jet numbers equal between different brand carbs?
  8. aaronwilde

    aaronwilde Member

    I've had to replace my magneto coil 2 times in a month scince i got it. I thought that water got in and ruined it.. still haven't got an answer as to wether or not liquid can ruin a magneto coil. My 3rd magneto just stopped working now too. I was wondering what this "cleaning the metal that is used for a ground path" thing is about? What do you mean "bad ground"?
  9. jaguar

    jaguar Well-Known Member

    Electrical current flows in a circuit loop which on motorcycles the "ground" part of it is the engine metal and often the frame. The current that flows to the spark plug goes from the ground side of the high voltage coil, thru the engine metal, and to the spark plug. That is one half of the loop. The other half is the hv coils secondary winding, the spark plug wire, and the center of the spark plug.
    If the spark plug is not tight then it has a bad "ground" connection which creates resistance to electrical current flow. If the stator coils ground wire terminal is not screwed tight against cleaned metal then it may have a bad connection which introduces resistance or makes a completely open circuit which causes ignition system failure.
  10. aaronwilde

    aaronwilde Member

    thanks for clearing that up. makes perfect sense :). i noticed on the head of my engine where the spark plug is screwed in is very black from oil. im going to go clean it right now and the washers and bottum screw in end of the spark plug is a bit dirty with oil too. would that affect the circuit loop?
  11. jaguar

    jaguar Well-Known Member

    it would have to be really bad to affect it. depends on the tightness of the spark plug mostly. I clean that area when I remove the spark plug. better safe than sorry
  12. the real causes of ignition failures

    Here is why you have failing parts in the ignition .
    Some of the Chinese manuals show to incorporate the white wire into the ignition system.
    the white wire should be cut off and thrown away. it is a generator line only! not an ignition wire. It will wipe out the cdi and sometimes it wipes out the mag too.

    also....the blue wire is the HOT.... the black is ground and is usually screwed onto the magneto frame
    use the black to the engine block for ground
    the only wire needed on the mag is the blue wire!
    you can remove the white wire and instead of the black going to the mag frame and route the black from the coil to any good ground point on the bike
    the only connection that HAS to go into the mag is blue to blue. Your kill switch just has to break the blue wire . Keep the black as ground.

    Here is a primer on how to electrically test you magneto and cdi.

    Disconnect the kill button to make sure it is not giving you a no spark condition. Some are defective. Make sure the plug wire is screwed securely into the CDI and you have a good connection at the plug boot, the plastic part that snaps onto the plug. If you've already done these things and have access to a good ohm meter check your ignition components against the numbers I've attached. Get back to us with the results.

    Use Low Ohm-Meter Scale About 200 Ohms

    1) Check Ohms between BLACK wire and WHITE wire. Reading should be around 2 Ohms

    2) Check ohms between BLUE wire and WHITE wire. Should be around 300-400 ohms.


    Use Ohm Meter High Scale About 200-K

    1) Positive lead on BLUE wire and Negative lead on BLACK wire should read infinite (no activity)

    2) Positive lead on BLACK wire and Negative lead on BLUE wire. Should be about 130-150 K-ohms

    3) Positive lead on Spark Plug wire and negative lead on BLUE wire. Should read between 135-155 K-ohms

    Switch to Low Scale 20K

    4) Measure between Spark Plug wire and and Black lead. Should be about 2.5 - 2.7 K- ohms
  13. aaronwilde

    aaronwilde Member

    I actualy just took the whole wiring set up apart. there was a white wire connected to the lower of the magneto coil but it wasnt connected at the other end. my kill switch was in between black and blue. now though, i have the kill switch totally un attached. Its just simply blue cdi to magneto, and black cdi to magneto frame ground. first thing i did when i got the bike was cut the ****ty boot off, strip the wire rubber and wind the copper cable directly around the spark plug and secure it into to nuts. so thats all good. I have replaced 3 magneto coils in a month scince getting the bike though. i took the magnet rotor off and it seems that the seal behind it could possibly be leaking a slight amount of oil when i got for long WOT rides.. which could be whats killing my magneto coil over and over? either that or the kill switch was? im seriously confused on how my coil dies so often.
    Last edited: Oct 13, 2011
  14. HeadSmess

    HeadSmess Well-Known Member

    now im :thinking: even harder....

    your using no boot at all? straight onto the plugs thread?

    hmm, very similar to my boot which is just a spring speared into the wire...

    no resistor.

    neither is the plug.

    what plug are you using? b5 or br5?

    this may be the reason to be using a boot and or plug with a resistor...

    ive only been having issues really since using this "spring boot"...

    just thinking... unquenched back emf...iunno...maybe...

    hmmmms. a plugs a cheap experiment...
    Last edited: Oct 14, 2011
  15. aaronwilde

    aaronwilde Member

    yeah i just took the wire from the cdi wraped it around the spark plug thread. i have 2 nuts that fit the sparkplug thread and the wires in between them and secure. and tapped too of course.

    im not sure which plug it is.. just the one it came with i presume (stock).

    my magneto isnt killing itself. tomorrow im going into the city to zoombicycles warehouse and im going to get a new cdi, magneto and probably the crank shaft seal that fits behind the magnet rotor as mine leaks small amounts of oil into my magneto area. cdi and magneto are 15 bucks each no taxes and the rubber seal is 2 dollars. cant really go wrong right?

    depending on how that goes i may even buy an expansion chamber exhaust as i hear it really makes a performance difference.. maybe even a better carb as mine leaks a bit of oil. i checked my magneto with a multi meter and it reads .002 ohms from white to black which is right on the 2k ohm setting. but then blue to white wire has no reading so i think its fried. oh well
    Last edited: Oct 16, 2011
  16. HeadSmess

    HeadSmess Well-Known Member

    i read "stock chinese plug"

    "purchase new cdi n magneto"

    something about a seal...

    and not one mention of getting a new plug! or a real ngk boot with a 5k resistor from a motorbike....

    bah humbug. last time i think for someone :shout:

    (the bpr6hs has stopped my speedo playing up from the interference too)
  17. aaronwilde

    aaronwilde Member

    ill get a plug too but should i get one from zoombicycles? itll probably be another chinese HT one. the new cdi im getting from zoombicycles also comes with a boot.
  18. BRIGUY38

    BRIGUY38 New Member

    Magneto protection