XS750 Triple - Ignition Dysfunction

[luckydevil]

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.

 

[Signal]

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.

 

[jetmechmarty]

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.

 

[Jan_P]

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

 

[Dougeast]

Did you get this figured out? I have the same (similar) problem.

Doug

 

[luckydevil]

Did you get this figured out? I have the same (similar) problem.

Doug

Hi Doug, sorry for the late response. I had a bad TCI unit. Bought a replacement and the replacement was bad too. The lesson for me was don’t assume your replacement part is working, especially if the part is difficult to test for function like the TCI unit. One thing that helped me was to isolate each component of the ignition system and test for function and continuity. It can be a tedious process, especially if you’re not an expert, like me. I took a blank page in my maintenance log and drew a flow chart of each step of the ignition process beginning with the switch/kickstarter and ending with healthy idle.

Feel free to message if you have questions, sorry I haven’t been tuned in.

 

[Dougeast]

No worries.. I got it figured out. Multiple things were wrong… timing way off (replaced the points and condensers and retimed), weak spark due to low voltage at the coils 10.2 V vs 12.8 at the battery (added the relay mod to power the coils), carbs would not sync (cleaned and set to factory settings). Still had an issue which eventually traced to a leaking float in the #3 carb. All sorted now and runs great.

 

[Dougeast]

No worries.. I got it figured out. Multiple things were wrong… timing way off (replaced the points and condensers and retimed), weak spark due to low voltage at the coils 10.2 V vs 12.8 at the battery (added the relay mod to power the coils), carbs would not sync (cleaned and set to factory settings). Still had an issue which eventually traced to a leaking float in the #3 carb. All sorted now and runs great.