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How to Set the Idle Up on a Mercruiser Boat Engine

Updated April 17, 2017

The Mercruiser out-drive motor, also called the stern drive, comes in a variety of cubic inch sizes for different size boats and applications. Many of them have two-barrel carburettors, while larger engines have the four-barrel variety for extra load and fuel demand. Rochester remains a common brand name carburettor used on Mercruiser motors, although other makes and models have seen service on the marine engines. A boat owner, with the help of an assistant, can set his carburettor up for maximum idle performance by following some simple steps and using a few basic tools.

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  1. Tie the boat to a dock. This procedure should be performed with the engine running and under a load while in the water, so prepare to cast off with enough fuel in the tank and tools on board. Pull back your engine cover. Untwist the knob that holds the plenum case over your engine and pull the case off. You can remove the spark arrester element, if you wish more clearance to access the carburettor.

  2. Start the engine and note the operation of the choke valve in the top of the carburettor throat. It should be fully closed, and then begin to open as the engine warms up. If it does not open all the way, use a screwdriver to loosen the three ring screws that hold the choke dial in place.

  3. Turn the dial counterclockwise to fully open the choke valve, but only a very small turn on the dial. If the choke valve was not fully closed, turn the dial clockwise to close it. Tighten the three ring screws. Shut the engine off. Hook one lead of a tachometer to the negative (-) lead on the ignition coil and the other lead up to a ground source on the engine or frame.

  4. Start the engine and note the rpm (revolutions per minute) on the gauge. Locate the two idle mixture screws that protrude from the carburettor base plate. Refer to your owner's manual for their location. Place a flex-handle carburettor adjusting tool on one of the idle mixture screws. Cast off and have an assistant place the shifter in forward gear at idle speed. Have the assistant steer the boat through a safe channel while you make the carburettor adjustment.

  5. Turn the mixture screw clockwise until the rpm on the tachometer gauge begins to fall. Stop. Turn the screw counterclockwise until the engine rpm begins to fall. Stop. Turn the screw clockwise until you reach the highest rpm number on the tachometer gauge. Perform the identical adjustment procedure on the second idle mixture screw next to it. Adjust the screw for highest rpm, according to the tachometer reading.

  6. Locate the idle speed screw on the carburettor cam linkage. You will see an outside throttle cam on the carburettor linkage, and the idle speed screw will rest on it. Refer to your owner's manual for the manufacture's recommended idle speed for your engine. For example, it could be listed as 750rpm for your engine.

  7. Have your assistant place the shifter in neutral. Use a slot screwdriver to turn the idle speed screw clockwise to increase the engine speed. Turn the screw counterclockwise to reduce the speed. Set it according to your manufacture's specifications, while watching the rpm reading on the gauge. Stop the engine.

  8. Replace the flame arrester element if you have removed it. Place the engine plenum cover back on and screw the knob back into its seat. Unhook the tachometer leads. Start the engine and check it for smooth operation through several shifts.

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Things You'll Need

  • Mercruiser owner's shop manual
  • Screwdrivers
  • Carburettor adjusting tool (flex-handle)
  • Tachometer

About the Author

Chris Stevenson has been writing since 1988. His automotive vocation has spanned more than 35 years and he authored the auto repair manual "Auto Repair Shams and Scams" in 1990. Stevenson holds a P.D.S Toyota certificate, ASE brake certification, Clean Air Act certification and a California smog license.

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