During a compression test, what must be done with the throttle and choke?

During a compression test, it is essential to have the throttle fully opened. This action allows for the maximum amount of air to enter the combustion chamber, enabling an accurate measurement of the engine’s compression. When the throttle is open, the engine can draw in air freely, which reflects the true capability of the engine to compress the air-fuel mixture. This setup offers a realistic representation of how the engine will perform under normal operating conditions.

If the throttle is closed, it could restrict airflow, leading to lower compression readings that do not accurately reflect the engine's condition. Therefore, fully opening the throttle ensures that the measurements taken during the compression test are valid and useful for diagnosing engine health.