The Yamaha SR500 has been around for nearly half a century and, while its ignition system was well engineered, age is now becoming as important as mileage when diagnosing faults. The closely related SR400 presents many of the same challenges.

Over many years of repairing SR ignition systems, rewinding stators and manufacturing replacement CDI units, we have built up a considerable amount of practical experience with these machines. We have now used that experience to completely review and update our Yamaha SR500 and SR400 Ignition Troubleshooting Guides.

The new guides take a systematic approach to fault finding, beginning with the simple checks before moving on to the HT coil, CDI unit and the ignition windings within the generator. We also look at an increasingly common problem with older machines: components from different versions being mixed together. SR stators, rotors and CDI units may look similar, and in some cases physically interchange, but that does not necessarily mean they are electrically compatible.

We have also expanded our advice on testing generator windings. Rather than simply accepting the very broad resistance tolerances sometimes quoted in workshop information, the guides explain how ambient temperature affects copper winding resistance and how temperature corrected readings can provide a much more useful indication of winding condition.

Our aim is not simply to identify a failed CDI unit. On a motorcycle of this age, successful diagnosis means considering the ignition system as a whole and establishing whether its individual components are still working together as Yamaha intended.

The guides are based on practical workshop experience and are intended to help owners and technicians diagnose faults methodically, avoid unnecessary replacement of good components and, most importantly, get these excellent old singles running properly again.