Rpm Loss

My 2003

Sundancer 380 is powered by two

370 hp MerCruiser 8.1 S Horizons. The port engine will lose 100 to 300 rpm while running at 3,600 rpm. It would stay down for a few minutes and then go back up to 3,600 rpm. It would continue this cycle if I ran at this rpm. I turned the engine sync off, thinking maybe it had something to do with it, but it continued to lose and gain rpm.

Tim Zwit Via email

Are the engines revving differently? (Does one sound different or is performance affected?) Or is it simply the gauge jumping? I ask because loose ground wires are notorious for causing tachometers to jump around. If the ground is good when the engine is cold, make sure all the spark-plug wires are securely fastened. A MerCruiser technician told me that when he hears of this happening on 8.1-liter engines, he immediately checks the spark-plug wires for cylinders 5 and 7.