Syncro or Tach- which is 'correct'?

[rmeitz]

I bought a 1990 310DA with 350's and V drives. The engines start and run great. There is an origional tach and a replacement tach in the dash. Up to 2000 RPM the syncro and tachs match. At 3000 rpm and above, I have to increase the engine with the 'origional' tach by about 400 RPM to get the syncro to center. I changed to the 'spare' props, but it didn't fia the problem.
I need some HELP with what is going on, and what to do.

 

[Redhook98]

Swap tachs and see if the symptoms follow (i.e. the different RPMs above 3000). Otherwise, hook up a portable tach (a timing light with built in tach works great), and see if either is bad. May have a stretched throttle cable also.

 

[BadaBingII]

I have owned three Sea Rays. All have had issues with tachs at one time or another.

I agree with Redhook, try swapping them . When you do, check for corrosion on the connections, and if the switches on the back of the tachs are in the same position.

If that doesn't do it, my default is to trust the syncro. Or better yet, just listen to your motors, the sound will tell you when you are in synch

 

[fc3]

Easy. Run the engines synchronized by the sync gauge and then by the tachometers. Compare the sound. When engines are running at different speeds, the noise harmonics are clearly audible. When the engines are turning at the same speed, the harmonics don't beat against each other and the sound is smooth and even.

 

[RonF]

Easy. Run the engines synchronized by the sync gauge and then by the tachometers. Compare the sound. When engines are running at different speeds, the noise harmonics are clearly audible. When the engines are turning at the same speed, the harmonics don't beat against each other and the sound is smooth and even.

Great explanation, Frank. Years ago, on louder boats, I could easily tell. It is harder now due to soundproofing and underwater exhaust. I'll have to try your suggestion myself.

 

[Presentation]

Easy. Run the engines synchronized by the sync gauge and then by the tachometers. Compare the sound. When engines are running at different speeds, the noise harmonics are clearly audible. When the engines are turning at the same speed, the harmonics don't beat against each other and the sound is smooth and even.

I agree.

As you are getting close you hear Woah…. Woah…. Woah.... then as you get even closer the speed of the Woah sound increase woah woah woah, when they are in sync you hear hmmm.

After you hear hmmm look down at the gauges to determine what’s correct.

 

[fc3]

I agree.

As you are getting close you hear Woah…. Woah…. Woah.... then as you get even closer the speed of the Woah sound increase woah woah woah, when they are in sync you hear hmmm.

After you hear hmmm look down at the gauges to determine what’s correct.

Nice job of explaining it!