2000 Carver 404 - Engine Sync Question

[Reardon]

Hi Fellow Boaters

I have a Carver 404 with twin Mercruiser 7.4 V8 engines (340HP). The engines have around 400 hours each and with the exception of a valve assembly rebuild on one piston on the port side have always preformed well. My issue is::

* Since I first bought the boat two years ago, in order to maintain a center line on my sync gauge, I have always needed to run the port side engine about 200 to 300 rpm faster.
* Performance wise, it does not seem to have any impact on the boats propulsion performance.

My questions are:

* Is this common and just part of two different engines that operate and are tuned differently?
* Do others out there always have the same rpm at the sync gauges centerline?
* Is the point of the sync gauge that in fact in order to sync you do need to balance the engines performance and they will operate at different RPM in order to be in sync?
* Any other thoughts would be great.

Engines are not exactly my thing, so any thoughts explained at a fifth grade level would be great

Thanks
Rich

[waybomb]

The synch gage is simply a finer tach reading.
If your synch gage shows you are in synch then the tachs should be equal. If not, either or is not reading correctly.
I think you are saying, for example, synch gage shows synced, yet one of the tachs is indicating 200 more rpm?

[Reardon]

Hi Way

Yes, the last thing you mentioned is what is happening. When the sync gauge is showing the engines are in sync, the needle straight up. When I am cruising at 17 knouts for example, then the port engine tach, is showing 4100 rpm and the starboard is showing 3800 rpm in order to maintain the sync gauge needle being straight up. So, I am running the port harder in order for the gauge to be centerline.

300 rpm variance does not seem like much (of course, what do I know) of a variance and there is no impact that I notice on the propulsion performance, sound of the engines. etc. Perhaps it is a calibration that needs to be done to one of my tach gauges or my sync gauge.

As mentioned, the port side engine was worked on pretty extensively a few months ago, as I had a valve assembly rebuilt in one of the pistons, but this was happening before that at about the same magnitude. When they rebuilt the valve assembly, I know they tuned, etc the port engine, so it has been went over pretty well.

Thanks
Rich

[waybomb]

You will need to hook up a portable tach and determine if the engine tachs are indicating correctly.

I am guessing one of your tachs is off and the synch gage correct.

On my boat the synch gage shows synch but my tachs are all over the place.

Viper suggested getting to the backside of the gages and turn the cylinder selector a few times in case there is corrosion built up.

On my current boat it's a pain to get to the backside. I have some work to do this winter so I will be checking connections and exercising the cylinder selection switches then.

Try it, may fix the issue

[mjk1040]

Rich; I was told to trust the sync and not the tachs. Mike

[Viper]

Source of the post ....Viper suggested getting to the backside of the gages and turn the cylinder selector a few times in case there is corrosion built up....

Pretty common issue and fix. Worth a try and doesn't cost anything but time. Check all connections at gages, and harness plugs under helm and at the engine. Corroded pins will cause erratic readings.

I agree with Fred in that a portable tach is useful because it doesn't rely on existing wiring which could skew the results. You can get a gage that hooks up electrically or a laser unit that counts revolutions of a piece of reflective tape you stick to the balancer. A good digital timing light will also give you an rpm reading.

[AaHubb]

In the mean time listen to your boat, you will hear a thrumming when the engines are out of sync.
..Aaron