You may hear both terms used in outboard ignition systems, but a trigger and a timerbase are essentially the same component. The word “trigger” is commonly associated with Mercury Marine, while “timerbase” came from Outboard Marine Corp. for Johnson and Evinrude applications.
This component is located under the flywheel and works much like a distributor does in an automotive application. The flywheel has a magnet located on the center hub, and the trigger / timerbase contains a series of sensors that detect that magnet as the motor turns.
When the flywheel magnet passes by those sensors, the trigger / timerbase sends a signal to the power pack / switchbox telling it when to release energy and which cylinder to send that energy to.
The trigger / timerbase also controls ignition timing on many outboard motors through mechanical advance. As you push the throttle handle forward, the throttle linkage rotates the trigger / timerbase so timing advances to match the rising rpm. If timing did not advance as throttle increased, the motor would usually fall flat on its face and die.
Because of that, it is smart to check the trigger / timerbase fairly regularly to make sure it rotates freely in its holder as the throttle is advanced. If it does not move freely, it should be cleaned and lubricated so it can operate correctly.
You should also inspect the linkages and connectors to make sure nothing is broken or damaged. A bogging motor can sometimes be misdiagnosed. Dirty carburetors are a common cause of bogging, but before tearing into the carbs, make sure the trigger / timerbase is not sticking and preventing the timing from advancing properly.
Catching that problem early can save both time and money, especially if the carburetors turn out to be perfectly clean.
