Setting Maximum Timing a 2.5 Liter Mercury Outboard

Here are the steps for setting ignition timing on Mercury 2.5 liter outboards. NOTE! This procedure assumes that there are no modules of any kind connected to the switchboxes (ie. "spark advance module", idle stabilizer, etc). Refer to the page that describes these modules for further information.

Tools Needed

Spark plug socket and ratchet
Timing Light
Remote Start Switch
Dial Indicator (Jay Smith sells these)
Spark Gap Tester
Screw Driver
15/16" socket and breaker bar

Procedure

Start by making sure that you have a fully charged battery.

Remove all spark plugs.

Insert the dial indicator in the #1 spark plug hole. Looking at the engine from the back, it is the top right-hand (starboard) hole.

With the 15/16" socket and breaker bar, slowly rotate the flywheel clockwise while watching the dial indicator. At the point where the needle changes direction stop. Slowly move the flywheel back-and-forth to verify the exact point where the needle stops to change direction. This is TDC (Top Dead Center).

Set the dial indicator to read "0" and check the pointer on the flywheel cover to ensure that it is pointing exactly on "0". Adjust it as neccessary.

Slowly rotate the flywheel counter-clockwise exactly 1 1/2 turns on the dial indicator. This is 25 degrees BTDC (Before Top Dead Center). Make sure that the timing mark on the flywheel indicates as such. Adjust as neccessary.

Go back and verify the "0" mark and the "25 degree" mark matches between the dial indicator and the pointer on the flywheel cover.

Remove the dial indicator from the engine, and re-install the spark plug in #1 cylinder only, and connect the plug wire.

Attach the remaining plug wires to the spark gap tester, and ensure that it is grounded. The spark plug wires can be grounded directly to the engine if desired. The point is to make sure that the remaining plug wires are all grounded!

Attach the timing light to the #1 spark plug wire.

Hook up the remote start switch to the starter solenoid between the largepost with red wires connected to it, and the post with the yellow/red wire connected to it.

Disconnect the throttle linkage from the throttle arm.

Turn the key on and ensure that the "kill switch" is not activated.

Hold the throttle arm wide open, crank the engine and observe the engine timing setting on the flywheel through the pointer on the cover.

If needed, loosen the jam-nut on the upper timing adjusting screw and adjust timing to 25 degrees max timing, then re-tighten the jam nut.

Note, the adjusting screw is shown as "2" in this picture.

Re-install all spark plugs and remove all test equipment.

Re-connect the throttle linkage.