KTM 125cc 2 Stroke Dirt Bike No Start

Hey Everyone,
I was wondering if anyone here has scoped a 2 stroke dirt bike ignition system? This is a kick-only race bike with no battery. It has a stator/pick up, cdi module and ignition coil. I have spark but it seems intermittent and very weak and won't jump much further than maybe .050" outside the cylinder. With the Pico scope I can see there's rarely a spark line visible even with the plug (or a gap tester) outside the cylinder. The coil can generate about 10kv but almost never jumps any kind of a gap. I found specs and ohm tested the coil and it seems to pass ok. Resistor cap and wire are also within spec. My problem is that I'm not sure what the input to the coil should look like and I wonder if I can get a primary waveform? The coil has 1 wire input and a ground that's shared with the cdi box. I put my old vantage on the disconnected coil input and set the scale to ac volts. Kicking the bike over swiftly produces 50vac spikes which I assume are meant to be coil firing events. Being that this isn't a standard 12v coil and distributor system I was a little hesitant to put my Pico's attenuated probe on that wire because I have no idea how much voltage that thing is supposed to generate or if there are any fly-back voltage spikes I need to be concerned with? I was looking to verify my input to the coil looks ok so I can eliminate the pickup and cdi unit as a concern if I can. I think if it's generating 50vac that should be more than enough to get a decent spark out of that coil but my goal here is to gain knowledge of the system so I can come up with an easy way of testing for no/weak spark on these old 2 smoke engines that I can repeat reliably in the future. Anybody have any input on this one? Thanks!

I haven't scoped a dirt bike ignition before, but an ignition coil is an ignition coil. It takes power and ground to create a primary magnetic field. When that field collapses, there will be a voltage spike ON THE CONTROL WIRE.  This is your primary voltage waveform.

The coil has 1 wire input and a ground that's shared

If the ground is shared and constant, your control is, likely, the power.

My problem is that I'm not sure what the input to the coil should look like and I wonder if I can get a primary waveform?

Be prepared for a voltage spike of around 400V DC and put your scope on it and see what you get. I'm not sure what the turn on will look like, but the turn off should be your primary waveform.

Leave your scope DC coupled.  Your scope can still capture AC waveforms when, DC coupled.

I'm sure you know this but, whatever you do, DO NOT let the secondary, or spark plug, voltage into your scope, directly.  To see a secondary voltage waveform, use an inductive probe around the plug wire.

 

Thanks Chad. It's a capacitive discharge ignition (CDI) so the control wire is the discharge of a capacitor across the coil primary to ground. There is no battery so the stator has to generate the power to charge the capacitor and then a trigger discharges it at the correct time. After some more testing and a bunch of research I've come to the conclusion that the scope isn't going to be of much help on this one. The problem is that the power generated is greatly effected by the speed of the engine, so trying to kick start this thing and get a reliable reading is impossible. I do have spark, just not enough to fire the plug in the cylinder. I have another bike with a similar ignition system so I swapped the coils. They ohm test about the same. The coil from the non-running bike jumps a huge gap on the running bike and the coil from the running bike puts out garbage on the non-running bike. So I think the coil is good. I ohm tested the stator and trigger and checked both to ground and they're also good. My thinking was that the stator isn't putting out the power to charge the cap and it's probably been sitting in that hot, rattley 2 stroke engine for the last 20 years. But it ohms out ok like the coil. The next thing I checked was the CDI module. I can't really find a reliable test for it but I did find that the steering stop on the left side is missing and it allows the fork leg to smack into the module and it's cracked in the back. The wiring and connections all look good so I'm pretty sure that's gonna be my bad guy. The problem is that it's the most expensive and difficult to find part in the system and I'm not sure this bike is worth it.

Thanks for the help!

*Btw, I had this long winded reply written out for you yesterday but when I went to post it the forum had logged me out and I lost the whole thing. Today it did the same thing but I was smart enough to copy it first. What's up with that?

Thank YOU. I did more learning than helping, in this thread.  Thanks for the update and info.