Why is my engine not getting to RPM?

Check the engine RPM and propeller shaft RPM with a tachometer.  Do this tied off at the dock, with the transmission in gear at idle. Check the engine RPM and the shaft RPM at the same engine speed.  Engine RPM/shaft RPM = gearbox ratio.

Check the engine injectors, compression, fuel supply – clean filters and correct micron filters, fuel supply and fuel filters.

Check air supply – clean air filters.  The air we breathe is about 20% oxygen. Diesel engines need huge amounts of air to get the oxygen needed to fully burn the fuel.

An engine starved of oxygen may not start or may exhaust partially burned fuel as black smoke or unburned fuel as white smoke. At maximum RPM, a 30HP naturally aspirated engine (no turbocharger) needs 3720 cubic feet of air per hour – 6.5 million cubic inches per hour.

Black smoke is caused from dirty fuel, plugged air filter, plugged exhaust, kink in exhaust hose, excessively high ambient air temperature, defective injectors, injection timing advanced or delayed, too much fuel injected, heavily fouled bottom or propeller, an over tightened stuffing box, a blocked turbo or a turbo with a defective smoke limiter.

Check the throttle system.  Throttle controls – adjusted for full throw.

Check Exhaust Pressure – Make sure that the Exhaust Elbow is not restricted.

Check for growth on the propeller, growth on the boat bottom.   Marine growth on the propeller and hull will increase hull resistance bringing down the RPM.

Tachometer

To perform a tachometer instrument calibration, verify that the high idle (wide open throttle) in neutral reaches the RPM specified by the engine manufacturers specification. This will confirm that the tachometer is reading correctly. Typically the tachometer gets its input from the alternator, which puts out a pulse that changes with the engine speed. The faster the engine turns the faster the pulses speed up.  If you have changed your alternator your new replacement alternator may supply a faster pulse signal at a given engine RPM than the original equipment alternator.

If the tachometer is not reading correctly, then it may be necessary run a strobe light on the shaft to verify RPM at wide open throttle.