Brad The Bike Boy: Low voltage at ignition units and coils

Saturday, July 4, 2026

Low voltage at ignition units and coils - Carb model SS and Monster

I had a carby 900SS in that was hard to start and could run funny at times. Although it clearly had carb issues, based on another 900SS I had recently with similar-ish issues - FCR carbed big bore engine that was hard to get to fire after sitting for a while - I checked the voltage at the coils / ignition units with the ignition on and not running. It was 9.3V, which was 2.5V less than at the battery.

To fix this, I fitted a relay triggered by the original power supply wire and modified the little loom to run power from the relay and earth it all back to the battery directly. Seemed pretty happy after that (then I got into the carbs to fix them too).

Photos show the install.  In this case, CA Cycleworks Exactfit 2VCOILS had previously been fitted (not by me), and they're upside down to how I'd normally do it.  But it did mean the relay fitted in there very nicely indeed.

Given these carby models are now all at least 25 years old, it's something worth checking.

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