During a compression test, what must be true?

During a compression test, it is essential to ensure that the throttle and choke are opened fully to allow the engine to draw in the maximum amount of air. Opening the throttle fully provides an accurate reading of the cylinder’s compression by simulating normal operating conditions. This is crucial because a closed throttle can restrict airflow, leading to artificially low compression readings that do not truly reflect the engine's health.

While other factors can be considered during testing, such as the engine’s temperature or the addition of oil to cylinders, they are not universally necessary for obtaining a valid compression test result. Focusing on fully opening the throttle ensures that you gauge the engine's capacity accurately and can better assess potential issues, such as valve sealing or piston ring integrity.