All starters wear down or burn out after a period of use. With any kind of marine engine, like a Mercruiser, it is important that you either have the original starter rebuilt, or purchase a OEM starter. Auto starters will work for a short while with a marine engine, but a car's starter is not meant to take the same kind of abuse a marine starter is. You can remove a Mercruiser starter with minimum effort and bring it to a shop for a rebuild, and avoid the cost of a new one.
Disconnect the negative terminal of the battery.
Open the engine compartment. Some Mercruiser engines will be built into a compartment similar to a car, while smaller outboard types will have a bonnet covering the engine that you can remove by releasing the spring clips on the side down near the propeller shaft.
Locate the starter. A Mercruiser starter will look similar to the starter in your car. It will have a large cylindrical bottom with a smaller one welded on top. An easy way to locate your starter is to follow the positive wire from the starter coil. The wire will end with a connection to the starter.
Tear off three small strips of electrical tape. Use the tape and a permanent marker to label the wires that attach to your starter. There is the positive wire (that comes from the starter coil), the ground (which will go from the starter to the engine block), and you may have a third accessory wire. Label them so you can remember where they are attached on the starter (top, centre pole, side pole).
Disconnect the wires from the starter.
Remove the locknuts from the bolts holding the starter in position. Most Mercruiser starters have bolts that will run from the bottom to the top, rather than from side to side like on a car starter. Once the nuts are off, reach underneath and pull the bolts out.
Remove the starter from the engine compartment by lifting it out.
If the locknuts on your starter bolts are not turning, use an additional socket wrench and lock this to the bolt nut underneath the starter. Some designs use a double threaded bolt and there will be a locknut on each end. You will have to "break" both locknuts free in order to remove them.
Never pound a bolt out. If you cannot pull the bolt free, spray the bolt with penetrating oil, tap the bolt lightly with the head of your socket wrench and let it sit for 10 minutes. Tapping the bolt will create a vibration in the metal that will work the oil down the bolt length. After 10 minutes you should be able to pull the bolt free.