Spark only at the beginning of cranking By adseguy (Read 3,388 times)

[adseguy]

I have the strangest issue guys. I only get one, sometimes two sparks at the beginning of a new crank and then it stops. It's very consistent. Crank, One (or two) spark(s), then nothing, let off, crank, one(or two) spark(s), then nothing.

Here are my measurements:

CDI output to coil:
.8 - 1.6 VAC
52Hz


Pickup sesnsor to CDI:
.25 - .32 VAC
380-480Hz (not sure if relevant)
388Ohms (within spec)


Primary Side Coil
.3 - .6 Ohm (slightly out of spec, but my multimeter leads read .2-.3 Ohm when touched together)


Secondary Side Coil
8.07 kOhm (within spec)


Cap
9.03 kOhm (within spec)


Spark plug tip to tip
4.75 kOhm


Stator
837 Ohms (within spec)
89VAC


I'm at a serious loss on what to do next as I've just replaced the CDI and still have the exact same issue. I've built engines before AND I'm an electrical engineer by trade....this one is getting to me



Let me know what you think. I can go verify or check anything else.



THANKS!!

[jct842]

Very possible coil is breaking down. Doesn't hurt to have a spare spark plug either especially if it is a Chinese one you are using. Intermittent problems are definately the hardest ones there are to diagnose. Most of the times I have ran into them when I was working for a living were only solved by substitution. When you test you are not testing under the same as a working condition. At one point on my old 150 I had a bad loose connection at the coil, a push on spade type.

[cyborg]

My money goes to bad coil

[adseguy]

Coil was my first thought too, but given that I get a good spark right from the start and EVERY SINGLE TIME I restart the crank process it led me to think that the CDI just fires away at the first crank signal then goes off the timing from the pickup on the stator. This to me would sound like the coil is fine, but the sensor or CDI was bad. Any more thoughts from anyone? I've already bought a $100 CDI and I'm trying to not buy things without good deduction. I know, I know it can be hard to diagnose without swapping the right parts, I'm just trying to make a good compromise. It's a 2004 Yamaha Vino 125 BTW.

Thanks for the replies so far, I'm looking forward to more.

[rapidjim]

I have had the same issue on a customers machine and it was the coil

[adseguy]

I just received the new coil today and the issue persists. What is next guys? Pick up coil?

[adseguy]

I FIGURED IT OUT!!

The main switch (key switch) was not properly shorting out the ground for the CDI, so the CDI indeed had power, but no ground to go off of. My error was assuming that because I was able to run the starter that means the continuity of the power signal was good. This indeed was not true as they are two separate lines that NORMALLY are shorted together at the main switch. I quite literally hot wired it once I found out the necessary short wasn't there. Nice big continuous spark now

[cyborg]

Great find!!!!!!! There's a piston right there