Tachometers are reference instruments: at a given engine speed, you should always know what to expect. If a difference appears, there's a problem, either with your instrumentation or with your boat's performance. Normally, a tachometer is allowed a 10 percent margin of error, and to discover how accurate your tachometer is -- or isn't -- you need a second reference instrument, either a shop tachometer or clamp-type multimeter with a tachometer feature.
Items you will need
Shop tachometer
Inductive clamp-type multimeter with tachometer function
Set the shop tach for your engine by selecting the number of cylinders and the correct number of times per second the ignition coil fires: For an 8-cylinder inboard, that's four pulses from the ignition coil; a 6-cylinder motor's coil pulses three times; a 4-cylinder motor, twice; and a single-cylinder gets one pulse per rpm.
Connect the shop tachometer's red lead to the dedicated gray shop tachometer lead on the motor or, if no shop tachometer lead is present, to the negative lead on the ignition coil. The shop tachometer's black lead is connected to any grounding point on the motor.
Start the motor, turn on the shop tachometer meter and run the motor up to 1,000 rpm on the shop tachometer. Take note of and compare this reading to your boat's tachometer.
Check the outboard's charging system if no signal is detected. The two tachometers should read within 10 percent of each other -- if the shop tachometer reads 1,000 rpm, your boat's tachometer should read between 900 rpm and 1,100 rpm. If your boat tach reads outside of that 10-percent range, high or low, and the shop tachometer's connections and settings are proper, you have a problem with your tachometer.