How to tune an outboard motor

Written by chris stevenson
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How to tune an outboard motor
An outboard tune-up will come in handy just as spring begins. (fishermen image by pearlguy from

Proper compression, fuel and spark play the major roles in outboard engine performance. Outboard engines do not get the chance to coast like automobile engines; outboard motors run under load almost all of the time, putting great stress on the ignition system components, pistons, rod bearings, rings and valves. Many outboard motors are stored seasonally and lie dormant for extended periods of time. Tuning the outboard motor for optimum performance will ensure reliable engine performance and trouble-free starting.

Skill level:

Things you need

  • Outboard motor repair manual
  • Socket set
  • Screwdrivers
  • Flywheel strap
  • Flywheel puller
  • Ignition points
  • Ignition condensers
  • Torque wrench
  • Feeler gauge
  • Spark plug
  • Drain pan
  • Oil filter wrench
  • Oil filter
  • Engine oil
  • Air filter
  • Gear case oil (pump bottle)

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  1. 1

    Activate the ignition cut-off switch and remove the key from the ignition. Remove the top engine case cowl to your outboard, by unclasping the case snaps. Set the cowl case aside. Place a flywheel strap around the outer rim of the flywheel and pull it taunt. Use a socket and wrench to loosen the flywheel nut.

  2. 2

    Place a flywheel puller over the flywheel and set the puller gasping hooks into the flywheel holes. Turn the flywheel puller bolt clockwise with a socket until the flywheel loosens from the shaft. Pull off the flywheel. Remove the ignition points case with a screwdriver. Pull the condenser wire, or wires, from the spring clip. Remove the two screws holding down the points. Remove two more screws, if equipped with dual points. Loosen each condenser screw with a screwdriver and pull out the condensers.

  3. 3

    Place the two new condensers in their mounts and tighten their mounting screws with a screwdriver. Set the new points on the base plate, then insert the screws and tighten them only enough to allow the points to swivel open and closed. Rotate the flywheel shaft until the high part of the shaft lobe touches the rubbing block on the points. Place a feeler gauge between the contact points and use a screwdriver to open or close the points by turning the screwdriver inside the adjustment slot.

  4. 4

    Refer to your owner's manual for the correct gap measurement for your points. Place the proper width feeler gauge between the contact points, so it moves up and down with some drag. Tighten both points screws. Perform the same procedure on a second set of points. Remember to rotate the flywheel shaft so the second set of points open, then slip the feeler gauge between the contacts for the proper gap. Tighten both screws with the screwdriver. Replace the ignition points cap and tighten it down with a screwdriver.

  5. 5

    Place the flywheel over the flywheel shaft. Replace the flywheel nut. Hold the flywheel still with the flywheel strap and torque the flywheel nut down to manufacturer's specifications, according to your engine repair manual.

  6. 6

    Pull the plug wire from the top of the spark plug. Remove the spark plug with a socket and wrench. Use a feeler gauge to set the gap on a new spark plug by inserting the proper gauge tip between the plug electrode (refer to your engine repair manual for the proper gap in thousandths of an inch). Screw the new spark plug by hand into the head. Tighten the spark plug with a torque wrench, according to specifications. Reconnect the spark plug wire.

  7. 7

    Drain the engine oil by removing the drain plug with a socket. Empty the oil into a pan. Use an oil filter wrench to remove the oil filter, unscrewing it counterclockwise. Smear a dab of oil on the new oil filter gasket ring and screw it up into its fitting. Tighten the oil filter hand-tight only. Tighten the drain plug with a socket and add the proper amount of oil to the crankcase through the dipstick filler tube, according to specifications.

  8. 8

    Use a screwdriver or socket, depending upon the configuration, to remove the air cleaner element inside the air cleaner housing. The housing will be fastened to the carburettor. Install a new air cleaner element inside the housing and reattach the housing to the carburettor. Use a socket or screwdriver to tighten the bolts or screws on the housing.

  9. 9

    Refer to your repair manual for the location of the fuel filter. It should be located inside the engine case, or in-line between the carburettor and fuel tank. Use a screwdriver to loosen the hose clamps and pull the old fuel filter out. Connect both hose ends to a new fuel filter and tighten the clamps with a screwdriver.

  10. 10

    Use a socket and wrench to remove the lower gear case filler plug. Check the level with your finger by dipping it inside. If low, add the proper type and amount of lower gear case oil, using a pump bottle. Fill the lower gear case to the top. Replace and tighten the filler plug with a socket.

  11. 11

    Place a 55-gallon drum filled with water under your propeller and lower unit. If you have a muff flush system, connect it to the lower unit water intake ports and fasten a garden hose to the muff flush. Turn on the hose with mild pressure. Deactivate the cut-off switch, replace the ignition key and start the engine. Refer to your repair manual for the location of the carburettor mixture and idle screws.

  12. 12

    Use a small screwdriver to turn the mixture screw inward or outward until you achieve the fastest engine idle. Adjust the idle speed screw to lower or raise the engine idle RPM (revolutions per minute). Shut off the engine. Activate the cut-off switch and remove the ignition key. Remove the water drum, or disconnect the muff flush system, if you have hooked one up.

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