Paul Eastham
Well Known Member
Thought others might be interested in this recent maintenance issue, didn't see anything like it in the archives.
Was on a short fuel run a few days back when I noticed something painted yellow on my Dynon. Cylinder #4 EGT was hot, 1500 degrees versus the usual 1200. Everything else seemed normal. Was almost home so didn't debug further, despite Mike Busch's admonitions...
On the ground, a mag check showed rough running on the right (Lightspeed ignition) and cylinder #4 going cold. Easy diagnosis, no spark, right?
On disassembly, spark plug tested good and wire tested good. Upstream of that is the coil, but the other cylinder that's fed from the same coil was firing normally, so the coil seems good...gotta be connectors I figured. After a bit of wiggling the wire around at different angles I saw the resistance spike and eventually cut out entirely.
I put in the order to Amazon for new cable and connectors and crimp dies -- and while waiting for that to arrive cut open the bad cable. The crimp doesn't seem to have fully captured the black/white conductor (see photo - barely), and when I poked around in the crimp a bit with my multimeter probe, a piece of thin wire broke off and fell out. Turns out these cables use a wound thin wire instead of a central conductor, and I guess it failed.
Interestingly, in the lightspeed manual, it says replace these cables every 10 years. Time on this cable: 11 years! FWIW, the cable was secured at two spaces along the engine pushrod, though I think some vibration probably happens. You'd think the larger crimp would protect this area ... maybe the cable was just defective from the start?
Was on a short fuel run a few days back when I noticed something painted yellow on my Dynon. Cylinder #4 EGT was hot, 1500 degrees versus the usual 1200. Everything else seemed normal. Was almost home so didn't debug further, despite Mike Busch's admonitions...
On the ground, a mag check showed rough running on the right (Lightspeed ignition) and cylinder #4 going cold. Easy diagnosis, no spark, right?
On disassembly, spark plug tested good and wire tested good. Upstream of that is the coil, but the other cylinder that's fed from the same coil was firing normally, so the coil seems good...gotta be connectors I figured. After a bit of wiggling the wire around at different angles I saw the resistance spike and eventually cut out entirely.
I put in the order to Amazon for new cable and connectors and crimp dies -- and while waiting for that to arrive cut open the bad cable. The crimp doesn't seem to have fully captured the black/white conductor (see photo - barely), and when I poked around in the crimp a bit with my multimeter probe, a piece of thin wire broke off and fell out. Turns out these cables use a wound thin wire instead of a central conductor, and I guess it failed.
Interestingly, in the lightspeed manual, it says replace these cables every 10 years. Time on this cable: 11 years! FWIW, the cable was secured at two spaces along the engine pushrod, though I think some vibration probably happens. You'd think the larger crimp would protect this area ... maybe the cable was just defective from the start?
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