Vintage Dirt Bike Q & A

I need some tips for diagnosing the cause of no spark on a bike I just assembled.

This is a bike I purchased that had a seized head. There was a piece of plastic in the cylinder and one of the bottom bearings had corroded stuck.

I rebuilt the bottom end, original piston and rings were fine - no over bore.

I have tested the kill switch - Oddly, it shows continuity when in the off position.

I have tried bypassing the switch, putting the switch to off and to on. No spark.

I do have a 76 running with a magento. My plan is to replace parts one by one to prove they are functioning and locate the cause of the failure. However, if there is a likely cause that can be easily diagnosed, I would like to try that first.

The kill switch shorts the coil input to ground, so your readings are most likely correct.

Try reconnecting the kill switch with the connectors exposed, set your meter to AC volts, and read across the connectors while kicking the motor over (switch on / spark plug shorted to ground of course). You should get a short pulse reading of some kind, probably less than 100VAC. If you get nothing, there's likely a problem under the flywheel (points/cam, stator coil, etc.) or in the wiring or coil.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric