4Strokes.com Tech: Testing for a Suspected CDI Problem - By qadsan
first thing to do is to look at all the wires in the electrical
system and make sure the insulation is not worn through,
especially the wires to the Capacitive Discharge Ignition (CDI).
Sometimes after years of use the wire insulation can wear off if
its rubbing against the frame and short out. It's also especially
important that the CDI itself has a good ground to work from,
otherwise it will not work properly. Double check to make sure
there's no corrosion on the ground connection of your CDI because
this can be a source of problems in itself.
I like to work from the source of where this all starts, so I'd start at the magnetic pickup and work from there. If you can get access to a scope, then you should see a small signal from the magnetic pickup as long as the flywheel is spinning. A multimeter set to AC should also show the voltage fluctuation if its set to a low enough scale. If I correctly recall, the peak voltage from the magnetic pickup is something like one volt or slightly less, but remember the magnet only passes the pickup coil once every revolution, which means that most of the time there's no voltage coming from the pickup sensor. It's this pulse from the magnetic pickup sensor that tells the CDI to fire a spark.
The CDI gets its power from the exciter coil on the stator and the voltage from this coil will vary with engine RPM, but it can be as high as 60-100 volts AC or more at higher RPM's, if I correctly recall. The exciter coil on the stator is usually the largest coil you can see. Some stators I've seen use two excitor coils, but the excitor coil(s) are usually significantly larger than the lighting coils and are very easy to spot when looking at the stator. The voltage supplied from the stator excitor coil is supplied through the CDI to one side of the ignition coil (the side with less windings on it). The ignition coil takes this AC voltage and steps it up to more than 10,000 volts and connects it to the spark plug through the spark plug's wire. The CDI unit is basically a fancy time delayed switch in that it sits there storing up electricity in a 'C'apacitor (hence the 'C' in CDI) until it receives a pulse from the magnetic pickup. Upon receiving this pulse, the CDI will then wait a certain amount time as pre-determined by the designed ignition advance curve before it 'D'ischarges (hence the 'D' in CDI) the stored electricity into the primary side of the ignition coil, which steps up the voltage via secondary windings and supplies it to the spark plug through the spark plug wire.
If there is electricity going into the CDI and coming out everywhere except for the spark end, and your test light shows no voltage on the wire going to the ignition coil, then the only logical conclusions would be that either the CDI unit is bad because it's not passing voltage to the primary side of the ignition coil or the magnetic pickup sensor isn't sending the required pulse to the CDI that allows it to supply the voltage to the primary side of the ignition coil (assuming the CDI is properly grounded, the voltages/signals check out, etc.). If you can somehow get a scope or decent multimeter to verify the output of the magnetic pickup sensor, then you should know what's causing your problem. Nowadays you can pickup an inexpensive digital multimeter for under $25 from many hardware and electrical stores.
Credits: Article by qadsan and editing by 4Strokes.com