Timing a Mercury 90 Hp Outboard Motor

by Contributor

Proper care and maintenance of your outboard motor is essential to its long-term use and functioning. As with any vehicle, there are minor adjustments that must be made occasionally to keep all of the parts working together to provide you a flawless ride. Timing a Mercury 90 Hp outboard motor is just one of the steps you can take extend the life of your boat and to maximize the pleasure you gain from recreational boating.

Install a dial indicator into your number 1 or top spark plug hole.

Find the TDC (top dead center) indicator and turn the flywheel clockwise until your number 1 piston lines up with it.

Bring your dial indicator back to zero, then turn your flywheel in a counterclockwise motion until your dial indicator reads .550" before top dead center (BTDC). Now, turn the flywheel back the other way (clockwise) until your dial indicator reads .491" exactly.

Adjust the timing of your ignition at cranking speed. You do this by taking all of your spark plugs out and replacing them with a spark gap tool temporarily.

Detach the throttle cable from its power head. Make sure that you have your timing light connected to your top spark plug lead.

Shift your outboard into neutral, hold the throttle arm in idle, and start up your engine. Be certain to make note of the timing marks as you do this.

Make adjustments to the idle timing screw until you achieve timing of two degrees BTDC at cranking speed. Make sure your maximum advance screw is against stop.

Start your engine up again as you take note of the timing marks. This time, adjust the maximum advance screw to a maximum of 24 degrees BTDC.


  • The 24 degree maximum advance screw adjustment is applicable only to models prior to serial number B239242. The maxmum advance screw adjustment for models after serial number B239241 is 28 degrees.

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