Gone Outdoors

How to Tune Up a Mercruiser 5.0

by David Curtis

The Mercruiser 5.0 liter engine produces 220 propshaft horsepower. This engine will give you years of trouble free service when properly maintained. It is recommended that a tune up be performed every 100 hours of service in fresh water and every 50 hours when used in salt water. Newer versions of these engines use the Mercury Thunderbolt ignition system providing quick reliable starts. You can tune your marine engine yourself by obtaining the necessary supplies from your local Mercury Marine dealer.

1. Remove the propeller using a Mercruiser prop removal tool. Inspect it for damage. Rub bearing grease over the propeller shaft and replace the propeller.

2. Open the engine cover and examine the belts and hoses. Replace the belts if they appear worn or cracked. Squeeze the hoses with your hand and replace them if they feel soft.

3. Remove the dipstick and attach a marine oil pump by screwing the end of the pump to the threaded base on the dipstick opening. Place the end of the dispensing hose into a container and pump until all of the oil has been removed from the engine. Disconnect the pump and replace the dipstick.

4. Remove the oil filter by placing a filter wrench over it and twisting the filter counterclockwise from the mounting base until it is loose. Remove the wrench and spin the filter off with your hands. Dispose of the old filter properly.

5. Remove the new filter from the box and rub some clean engine oil over the gasket with your finger. Spin the new filter on the base with your hand and tighten it one full turn after you feel the gasket meet the mounting base.

6. Open the engine fill cap on the valve cover and place a funnel inside of it. Fill the engine with new oil until the dipstick oil level is at the full mark.

7. Find the fuel filter between the fuel tank and the carburetor. It looks similar to the oil filter. Use the oil filter wrench to remove it the same way as the oil filter. Replace the fuel filter with a new unit.

8. Examine the rotor cap and make a diagram of the spark plug wiring connections. It is critical that the cables are replaced in the same sequence for the proper firing order. Remove the plug wires from the cap by firmly pulling them off with your fingers. Push back the retaining straps on the distributor cap and remove it. Check for any deposit build up or corrosion on the inside. Remove the rotor by pulling it upwards and replace it with a new unit. Replace the distributor cap with the new one by placing it on the base and clipping the straps back over the edges.

9. Remove one of the spark plug wires by pulling it off with a spark plug wire tool or using your fingers. Remove the spark plug with a socket wrench. Remove the new plug from the box and check the gap of the plug with a gapping tool. Consult your owner’s manual for the proper measurement for your particular engine. Adjust the gapping as necessary and install the new plug. Be careful to only snug it down and not to over-tighten it. Compare the plug wire you have removed with the new wires in your kit and replace it with one the same length. Connect the other end to the distributor by using your diagram for the proper location. Repeat the process until all of the wires and plugs have been replaced.

10. Remove the spark arrestor located on top of the carburetor. Remove the retaining nut using an open end wrench. Place the unit inside a large flat container and pour enough mineral spirits to cover the fins. Allow it to soak for 10 or 15 minutes and remove it. Blow dry the fins with compressed air and reinstall the arrestor.

11. Remove the battery cables and clean the posts and terminals with a wire brush. Reconnect them and give the battery a full charge.

Items you will need
  • Socket wrench set
  • Open end wrench
  • Flat screwdriver
  • Containers
  • Engine oil pump
  • Distributor cap
  • Rotor
  • Paper and pencil
  • Spark plug gapping tool
  • Spark plugs
  • Spark plug wires
  • Fuel filter
  • Oil
  • Oil filter
  • Bearing grease
  • Mineral spirits
  • Compressed air
  • Battery terminal brush

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