Assessing Used Outboard Engines

by James W. Hebert

Outboard enginess have a long life and are often sold and purchased in a used condition. The best assessment of a used engine can be made by actually operating it on the water. Further insight into the condition of the engine can be made by a close inspection and compression test. A procedure for performing a quick inspection and compression check is presented below. The procedure is based on one suggested in an 1992 OMC Service Manual.

Inspection Procedure

Visual Inspection

Visually inspect the engine for leaks, missing or loose parts, and obvious defects.

Compression Check

Proper compression is essential for good engine performance. An engine with uneven compression between cylinders cannot be properly tuned.

If the engine has equal compression, is hard to start, AND operates poorly, check for:

If the engine shows a variation greater than 15-PSI (100-kPA), check for:

If the engine has equal compression and runs normally, continue the inspection procedure.

Spark Plugs

Clean and re-gap or replace the spark plugs. Make certain the ceramic portion of the plug is not cracked. Replace if cracked. Verify the use of the proper spark plug type and gap. This information is usually found in the owner's manual, although in many instances a manufacturer will change the spark plug recommendation after the publication of the owner's manual. Check with a competent factory authorized service facility to determine the current recommendation for spark plug type and gap.

Ignition Components

Inspect and test all ignition components for signs of damage. Inspect all wiring for deterioration or wear of the insulation. Check spark plug boots. Inspect the under-flywheel coils (if visible) for signs of over-heating or damage.

Fuel Components

Inspect all fuel lines, hoses, filters, pumps, etc., for signs of leaks. Check for deteriotated rubber and gaskets. Remove air silencer. Inspect carburetor linkage. Check for synchronizaton of throttle plates. Pressurize fuel system and check for leaks.

Lower Unit and Propeller

Inspect the propeller for damage. Remove propeller. Inspect the propeller shaft seals for signs of damage.

Gear Case Lubrication

Drain gear case lubricant. Inspect for signs of water intrusion. Refill gear case with fresh lubricant.


Check carefully for any loose fasteners on the engine. Check the cylinder head bolts for proper torque.

DISCLAIMER: This information is believed to be accurate but there is no guarantee. We do our best!

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Copyright © 2005 by James W. Hebert. Unauthorized reproduction prohibited!

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Author: James W. Hebert
This article first appeared February 11, 2006.