The base timing should be set to (factory recommended) 10degrees BTDC with timing light and can be checked / set at any RPM below 2,000rpm in this diagnostic mode.
Hint: For large cams or lumpy idle applications, raise the engine RPM to 1,500rpm with an assistant or feeler gauges under the throttle stop screw to reduce timing “shake” while setting it. Remember to keep RPM below 2,000rpm while setting the timing.
You can check you are correctly in the diagnostic mode if there is no CEL etc. by testing that: above the 2,000rpm
point, the timing will change from a fixed value 10Deg BTDC to normal variable timing of more than 30Deg BTDC.
As the RPM is reduced to below 2000rpm again, it will return to the fixed setting value that should be 10deg BTDC.