How Do I Check RPMs on My Boat Motor?

When your boat motor operates, its crankshaft turns many times each minute. The number of revolutions per minute the crankshaft makes is a very general indicator of the motor's overall health. The shop tachometer can reveal, visually, that sense of hesitation, a surge in the way the motor runs, or an uneven idle. A marine shop tachometer may even give a more accurate reading of the rpm your motor produces, because its not part of the motor.

Things You'll Need

  • Marine shop tachometer
  • Propeller wrench
  • Flushing attachment
  • Garden hose
  • Outboard motor test tub
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Instructions

    • 1

      Set the shop tachometer to the two-cycle setting if your motor is a two-cycle motor. If your motor is a four-cycle motor, set the shop tachometer to the four-cycle setting.

    • 2

      Inspect the numbering on the face of the tachometer. If the notation on the face of the tachometer says "x100," each number on the face of the tachometer equals 100 rpm. If the notation on the face of the tachometer is "x1000," each number on the face of the tachometer represents 1,000 rpm.

    • 3

      Locate the gray cable that comes from the ignition coil, for single-cylinder boat motors. Connect the shop tachometer's input cable to the gray cable. If you are unable to locate a gray tachometer wire, connect the tachometer's lead to the No. 1 spark plug lead.

    • 4

      Set the pulse rate on the shop tachometer. The shop tachometer is driven by the pulses from the coil or from the spark plug lead. There are two pulses for a four-cylinder powerhead, two for a three-cylinder, and one for a one- or two-cylinder powerhead.

    • 5

      Start the motor and allow it to warm naturally. When the temperature gauge indicates that the powerhead has warmed up, read the number on the face of the shop tachometer. Multiply that number by 100 or 1,000 -- according to the notation on the face of the tachometer -- to determine the number of rpm at which your boat motor is operating.