• Only sparks once when trying to start [IN-DEPTH TECHNICAL ANSWER INSIDE]

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 #10699  by hbr1009
 Tue Aug 01, 2017 3:35 pm
I have a problem with no spark on a Yerf Dog Rover CUV model 34800. It has a DC CDI box. I ran all the tests I could based on the no-spark guide on buggydepot. I have replaced the stator (including sensing wire), the regulator/rectifier, the CDI box, the switch key, the ignition coil/plug wire, and the solenoid. It was starting fine, then would start intermittently, now won't start at all. Now it has one spark initially when cranking then no more. Any help would be more than appreciated. I'll try anything. Thanks.
 #10700  by Travis @ Buggy Depot
 Wed Aug 02, 2017 1:01 pm
When I've seen this, it was due to a weak or malformed trigger signal.

You should check for pinched wires in the stator installation that may be crossing an AC signal from a charging winding to your trigger wire. Check under the metal cable clamp behind the stator. Also check to see if the wrong (longer) bolts were used to secure the flywheel fan, that will rip up the windings and can cause what you're describing.

Warning: The rest of this answer gets a bit deep into the technical side.

Here are couple pictures that represent a good versus bad signal. This was on the bench with a DIY trigger generator I made to test GY6 CDIs. So the signal is artificially square and continuous. If you look at the signal from a true trigger sensor, it will be sinusoidal and timed per each revolution.

Bad generated signal producing "One spark":
If the negative segment of the signal is blocked, you'll only get one spark.
Image

Good generated signal:
Note that the good signal has both positive and negative portions of the wave intact. A 12v (24vpp) trigger signal allows the system to produce the strongest spark. Less than that yields proportionately less spark. I think the threshold is 2v (4vpp). You won't be able to measure this with a multimeter, only an oscilloscope.
Image

A side note, it turns out our DC CDI for your CUV can reach 30,000RPM (thirty thousand, not three). No rev limit. That's three times faster than the engine can actually turn.
Image

Check for pinched stator wires
The only scenario where I think this might actually be the case, is if you are somehow getting a cross connection to one of the other AC stator circuits. I recommend that you check your stator installation for any pinched wires. It's a common install mistake, I've done it before myself.

After that, I'd be leaning more towards swapping out the CDI to see if it was damaged during troubleshooting, or perhaps arrived defective.
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