A 20-hour service on an outboard engine typically involves several maintenance tasks to ensure the engine's reliability and longevity. Here are some common procedures:

1. Oil Change: Drain and replace the engine oil to remove any contaminants and maintain proper lubrication. This includes replacing the oil filter as well.

2. Gearcase Inspection: Check the gearcase for any signs of water intrusion or damage. Inspect seals and gaskets, and replace them if necessary.

3. Fuel System Inspection: Inspect fuel lines, connections, and filters for any leaks or blockages. Clean or replace fuel filters as needed.

4. Spark Plug Inspection: Check and clean or replace spark plugs to ensure proper ignition and engine performance.

5. Cooling System Inspection: Inspect the water pump impeller and housing for any damage or wear. Replace if necessary to ensure proper cooling of the engine.

6. Anode Inspection: Check sacrificial anodes for corrosion and replace them if needed to protect the engine from galvanic corrosion.

7. Throttle and Shift Linkage Inspection: Inspect throttle and shift linkages for proper operation and lubricate as necessary.

8. Battery Inspection: Check the battery's condition and connections. Clean and tighten connections if needed.

9. Overall Inspection: Perform a visual inspection of the engine for any signs of wear, damage, or leaks. Check all nuts, bolts, and fasteners for tightness.

10. Propeller Inspection: Inspect the propeller for any damage or debris. Clean or replace the propeller as needed.

11. Performance Testing: Run the engine to ensure it starts smoothly, idles properly, and runs at all throttle ranges without any issues.

It's essential to follow the manufacturer's recommendations and guidelines for the specific model of outboard engine being serviced. Additionally, consulting the owner's manual or seeking assistance from a certified Yamaha technician can ensure that all maintenance tasks are performed correctly and thoroughly.