Which gauge to trust?

R

River Runner

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I have noticed that on my Gibson when I am cruising the port engine tachometer shows about 100 RPM slower than the starboard engine. The engine sync gauge shows them to be even. Which would you trust? The engine harmonics sound correct when the sync gauge shows them even.

Is a 100 RPM difference enough to worry about?

Thoughts?
 
If they sound in sync it may be the one tach is out of calibration.
 
I'd trust the sync gauge or an active synchronizer system.... or your ears.

I believe most analog and many digital tach have small calibration rotary switches separate from pole/pulse/cyl switches on the back.

You could use a digital advance timing light with tach , or inductive TinyTach or any good digital tach as test/reference tach on each engine and adjust each dash tach to agree. (Tiny adjustments.)
 
A digital tach us cheap and easy to use to calibrate all the tachs.
 
Trust the sync gauge and you ears. My starboard tach always sticks. A few taps of he glass moves the needle, but I have to repair it. There is a thread somewhere that shows you how to open it up and repair it. It's usually from moisture.
 
Easy way to check your tach. Paint a white stripe on your crankshaft pulley, then video it with your phone. Find out the frame rate and when the rpm is such the line stands still, you can verify the rpm.

https://m.youtube.com/watch?v=MKd6FZu-OW8
 
Thanks guys! I took the boat on a 6 hour cruise over the weekend and it ran flawlessly. I spent the night on hook overnight and was heading back to the dock when I noticed my Starboard tach was now about 1300 RPM higher than the port but my ears had not registered a change in engine note. Both of my throttle controls were centered with each other so my mind quickly went to a possible drive issue. As I reached up for the throttle to pull back the tach suddenly dropped back to 1900 RPM still the same 100 RPM above the port side. Then the tach suddenly shot back up again. It is strange how it would read fast versus slow or not at all. Oh well, I have now determined the culprit. Now time to figure out if it is a gauge, a distributor module, or a wiring issue. I am leaning towards gauge since they are most prone to weather. This problem is at the flybridge station and honestly I have not checked at the lower station yet since I have never operated from down there...yet. That is the next check. Unfortunately the problem does not happen all of the time nor regularly so by the time I make it from one station to the other the problem can be gone and back to normal.
 
Tach's often have a selector switch on the back - for use w/ 4cyl, 6cyl, or 8cyl engines (the tach is set once the engine is installed.) Mine takes a small flat-head screw driver to switch. I've found that the contacts in this switch corrode and/or loose a bit of conductivity over time, and cycling the switch between all settings seems to clean up the contacts. Just remember to set it to the correct engine once done!!!

good luck.
 
quote:
Originally posted by Good Grief

Tach's often have a selector switch on the back - for use w/ 4cyl, 6cyl, or 8cyl engines (the tach is set once the engine is installed.) Mine takes a small flat-head screw driver to switch. I've found that the contacts in this switch corrode and/or loose a bit of conductivity over time, and cycling the switch between all settings seems to clean up the contacts. Just remember to set it to the correct engine once done!!!

good luck.

Click to expand...


x2 . This is one of the 1st things the tach mfgrs. recommend trying when tach readings begin to seem "off".
 
Thanks guys. I'll check this out once the rain stops this weekend. I guess if the tach thought it was for a 6 or a 4 the RPM would jump up quite a bit. I hadn't thought of that one.
 
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