XS750 Triple - Ignition Dysfunction


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North Carolina
My #3 cylinder plug has an intermittent spark issue. Sometimes it won’t fire at all, sometimes it quits on me a mile into the ride and I limp home…doing the walk of shame past my dog. The same issue occurs when the ignition coil is switched, so it isn’t my plug wire or ignition coil. My plug wires are 5kΩ NGK paired with non-resistance NGK plugs. Ignition coils all read resistance to spec.

This triple isn’t like my ‘78 xs650 with points, it has an ATU (automatic timing unit) paired with a pickup coil unit, which signals a CDI “ignition unit” box, which tells my spark plug to fire (as far as I understand.

Could the pickup unit still be failing somehow, or does correct resistance (and continuity) ensure the coils are doing their job? I’ve already replaced the CDI box, but maybe the replacement is bunk. Any other troubleshooting I can do? I’m really trying to understand the problem, not just blindly replace parts and keep my fingers crossed.

As always, thanks for tolerating my 750 questions, and thanks in advance for the help!

Here’s a photo of the machine and the pickup unit.


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It does sound like the fault occurs when the system is at operating temperature. Leaning towards the coil or electronics.
You have swapped coils so its not them.
I suppose you have already cleaned all the connectors and checked the wiring for breaks.
To do a valid test of the TCI ignition unit by replacement, you need to use a known good TCI.
A crude test could be done by inducing the fault then spraying the TCI unit or sender coil with freeze it to cool it down.
A can of air duster (for computer keyboards) held upside down will work do the job. The propellant does the cooling.
Check with Rex's speed shop they may be able to test the units you have. https://www.rexs-speedshop.com/

If your units are faulty there are alternatives around so don't pay over the odds for a replacement stock TCI it ay be cheaper to replace with a new system.
On the XS1100, it’s the pickup coil wires that break. They break inside the insulation, so it isn’t visible. I had to fix it on my bike. Sorry, that’s all I have to offer.
Not familiar with the 750 Or how the ignition works
If it was me I would start check the charging Voltage -- First at home and then install a Voltmeter se what it reads out there
on the road .
Some systems ( Boyer ) can flip out if the Voltage is out of range To low it wont fire to high it can drop one cylinder
The transistors and so can behave erratically .
If that checks out I would control that it is correct going in at Coils and boxes so no drop ..downstream