When checking timing on older vehicles, what should be done with the vacuum line to the vacuum advance unit?

When checking the timing on older vehicles, it is essential to disconnect and plug the vacuum line to the vacuum advance unit. The vacuum advance mechanism relies on engine manifold vacuum to adjust the ignition timing based on engine load and speed. When the timing is set with the vacuum line connected, it can lead to inaccurate readings because the vacuum advance will influence the timing adjustment during the timing check.

By disconnecting the vacuum line and plugging it, you ensure that the ignition timing is set to the base timing without any contributions from the vacuum advance. This allows for proper adjustment of the timing to the manufacturer's specifications. Once the timing has been set, you can then reconnect the vacuum line, allowing the vacuum advance to function correctly during operation. Doing so ensures the engine runs efficiently and prevents potential timing-related issues while driving.

The other options do not provide the correct method for checking timing, as leaving the line connected can give skewed results, completely removing it can lead to a loss of function, and checking for leaks, while important for engine performance, is not directly related to setting the timing.