How to Set the Timing on a MerCruiser 5.0

Written by norm rogers
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How to Set the Timing on a MerCruiser 5.0
(Thomas Northcut/Photodisc/Getty Images)

Properly setting the timing on an engine, whether boat or automobile, is essential to the operation and performance of that vehicle. The valves must be in proper time to allow fuel and air to enter the cylinder and for exhaust gasses to escape. The pistons must be in proper time so spark occurs when the air and fuel mixture is compressed for most efficient combustion. This can all be accomplished by the simple act of turning the distributor to the correct position, i.e., properly setting the timing.

Skill level:

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Things you need

  • Tune-up kit (if needed)
  • Timing light
  • Distributor wrench

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  1. 1

    To ensure the timing is set correctly, you must first ensure that the contact points inside the distributor are set correctly. If the engine has not had an ignition tune-up in the past year, one should be performed before the timing is set. Attempting to adjust an old set of points with a feeler gauge may result in a bad gap setting. Purchase a tune-up kit containing the distributor cap, rotor, points and condenser at a local marine parts store or online.

  2. 2

    Inspect the timing marks on the engine pulley and the corresponding timing tab. It may be helpful to mark them with chalk prior to starting the engine. Connect a timing light in line with the number 1 spark plug and start the engine. Set the idle speed according to specifications found in the repair manual. Point the timing light at the timing tab on the engine. Take care to point the timing light straight to get a correct reading.

  3. 3

    If the correct mark on the timing tab does not align with the timing mark on the engine, loosen the bolt that secures the distributor and twist the distributor slightly until the correct marks align. Tighten the distributor bolt and recheck the timing to make sure it did not change. While viewing the timing marks, have an assistant accelerate the engine speed. Confirm that the timing mark advances, indicating the advance mechanism is properly working.

Tips and warnings

  • Because each engine is different, consult a MerCruiser repair manual to obtain the proper point gap setting, timing settings and other specifications. Setting the point gap is only necessary on engines not fitted with electronic ignition. See your marine parts dealer for the proper tune-up kit for your engine.
  • Never attempt to start or run the motor without a water supply connected to the raw water intake on the lower unit. Do not use parts for your boat engine that are not specifically designed for marine use. Some parts may appear to be interchangeable with automobile engines, but many parts are not. Substituting automobile parts for marine use can cause damage to the engine or lead to safety hazards.

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