Ignition Tune-Up on an Older Sterndrive or Inboard V6/V8

Write down the engine model and serial number from the engine's ID plate and look up the correct parts. Engines of the same size can use different distributors and ignition systems depending on the year, and the parts are not interchangeable. If your engine still uses breaker points and a condenser, those must be set with a special gauge and checked with a dwell meter afterwards. That is fine for experienced DIYers, but many owners have a shop do it or convert to electronic ignition.

Open the hatch, run the blower for several minutes and check for fuel smell. Let the engine cool so you don't get burned and so the aluminum plug threads in the head are not stressed. Disconnect the battery and remove the key.

Blow or brush away dirt and rust flakes from around each plug before removing it, so nothing falls into the cylinder. Label the plug wires with tape (cylinder numbers) if they are not already numbered.

Pull one plug wire by its boot, not the wire, then remove the plug. Working one at a time keeps the wires in the right order. Line the plugs up in order and compare them. Boating Magazine's guide to reading plugs:

• Deep brown insulator and no unusual deposits: healthy.
• Water droplets: a possible leaking head gasket or exhaust manifold. Stop and call a mechanic before running the engine.
• Specks of aluminum: possible pre-ignition damage. Stop and call a mechanic.
• Whitish insulator: a lean fuel mixture.
• One plug not "colored in" like the others: that cylinder may not be firing, from an ignition or fuel problem.

Black, dry soot usually means a rich mixture or too much idling.

Use only the plug part number in your owner's manual; Discover Boating warns that the wrong heat range can cause pre-ignition, which can melt a piston. Check the gap with a gauge against the spec in your manual, even on pre-gapped plugs, and adjust carefully if needed. Thread each plug in by hand first to avoid cross-threading, then tighten to the torque in your manual. On engines with plugs tucked under the exhaust manifolds, a low-profile spark plug socket makes the job far easier.

Look for cracked, hard or burned insulation, corroded terminals and boots that are brittle or loose. Wires that look fine can still be weak inside, especially on older engines. If replacing, swap one wire at a time, matching the length and routing of the old one, so the firing order stays correct. Keep wires away from the hot exhaust manifolds and clipped into their separators. Push each boot on until it seats firmly on the plug and on the cap.

Note the cap's position and which terminal goes to which cylinder (take a photo, and mark the number 1 terminal). Remove the cap's screws or clips without disturbing the distributor body, which would change the timing. Leave the wires attached to the old cap for now.

Inside the cap look for cracks, burned or corroded terminals, carbon tracks (thin dark lines between terminals) and a worn or missing center carbon button. On the rotor look for a burned or eroded tip and cracks. Moisture or a white crust inside the cap means water is getting in. Replace the cap and rotor as a pair if any of these show up, or as your manual's schedule says.

Install the new rotor so it seats fully on the distributor shaft in its only correct position. Set the new cap beside the old one in the same orientation, then move the wires across one at a time from the old cap to the matching terminal on the new one. Fit the cap, making sure any gasket or seal is in place, and secure it. A marine cap that screws down or seals is part of the ignition protection, so do not leave out screws, seals or vent screens.

Double-check every wire is pushed fully home at both ends and routed away from heat and moving parts. Reconnect the battery, positive cable to the positive post first, and double-check the polarity before you tighten anything. Mercury's owner FAQ warns that reversed battery cables can blow fuses and damage regulators, rectifiers, the engine computer, gauges and boat electronics; if it happens, have a dealer inspect the boat before you run it. Make sure nothing is left on the engine, such as tools or rags.

With water supplied to the engine (on the water or on a flush setup your manual approves), run the blower again, then start the engine. It should start readily and idle smoothly. A miss, a stumble or popping usually means a wire on the wrong terminal or not seated. Shut down and recheck the order. Never run a sterndrive or inboard without cooling water.

Timing sets when the spark happens and has a big effect on power, fuel use and engine health. Checking it needs a timing light, the correct spec for your engine, and on many engines a special procedure, such as putting the engine computer into a base timing mode. Wrong timing can cause detonation that damages pistons. Unless you have the service manual and experience, ask a mechanic to check the timing as part of the tune-up, especially if the distributor was disturbed.

Write down the date, engine hours and parts used in your BoatMend maintenance log. Keep a spare set of plugs on board, which Boating Magazine recommends along with the tools to change them.