What is the ignition timing for 1991 Chevrolet truck?

There's no single ignition timing specification for a 1991 Chevrolet truck. The correct timing depends entirely on the engine size and whether it has a distributor (older style) or computer-controlled ignition (more common in 1991).

You absolutely cannot determine the correct timing without consulting your owner's manual or a reliable repair manual specific to your truck's engine. The manual will specify the timing in degrees BTDC (Before Top Dead Center) at a certain RPM, and may also list different specifications depending on engine temperature or other factors.

Attempting to set the timing without the proper information could damage your engine. If you are unsure how to do this, take your truck to a qualified mechanic.