general question about engine sync

[dfg4240]

Four tachs on board plus sync indicators at upper and lower station.
the lower helm has a small box that has 3 lights and when the engines are in sync the middle one comes on (GREEN).
The upper one is the digital and flashes the word sync when engines are close.At idle the starboard engine always seems to be about 120 RPM lower also I always have to push the starboard side a little to match the speed of the port engine.
all four factory tachs read the same but not the digital sync.Should I get a tach and check them out. IT is not a big deal but I did notice after my trip from seneca lake that the port engine seemed to burn a little more gas than the starboard engine. As far as the engines go they run beautiful and quiet.
At idle the starboard engine reads 620 and the port says 710 according to the digital tach.
dfg4240

[captainmaniac]

Are you saying the synchs don't agree, or your synchs don't agree with your tachs? Your synchs are usually more accurate than your tachs. The synchs are just trying to tell you about the difference between the speed of the 'ticks' they get (when a plug fires), while the tachs are trying to convert the speed of those ticks into a reading. If the tachs are off, there is usually a calibration screw on the back of them to adjust.

[dfg4240]

thanks captainmaniac
all my helm tachs read the same-all four of them but my digital tac uptop always reads about 120 rpm higher on the port engine than the starboard engine.
If my engine tach reads 800 at idle-the digital tach reads 760 on the port engine and 660 or close to it.
I usually don't even look but adjust when I'm out and get to read sync. At idle the starboard engine always reads about 760 and the starboard reads in the 660 to 680.
I will have to look it up and see what it should be and adjust so they are even.
I should get a tach anyway-this way I will be sure.
dfg4240

[LandVF36]

I've always sync'd by ear. I had a neon strobe sync on a couple of boats, but honestly they drive me nuts. If you keep your engines in sync (by some gadget or by ear, and you have a different fuel burn rate between engines on a long haul, something is off. Check the easy stuff first like plugs, wires, cap/rotor... basic ignition stuff. Next, conditions of the props, tranny slip (clutches? change the fluid first). Then, things get a little more serious like checking compression, carb issues, etc.