How do I go about calibrating the tachometer in my boat?

I had the boat out by myself the other day and opened it up. I got 52MPH at about 5200rpm, I know marine speedos are notorious for not reading correctly, but what concerned me is the tach. The markings leave plenty to be desired on the OEM tach, only marked every thousand rpm, so I'm guessing at the 5200 rpm by looking at the needle position. I know the motor could not be running at 5200rpm because the rev limiter on my motor is set for 4950 rpm, 4400-4800 is where it should be WOT. So It's off by at least 250rpm. I would say it's off by 350 rpm or more because I don't think the engine was turning 4,800 rpm it definitely didn't sound stressed at all.

Before I do anything I am going to find the switch on the back of the tach and move it back and forth a few times to clean the contacts then take the boat out and try it again.

I have read about using a frequency generator and one guy used a fluorescent bulb and a piece of tape on the belt pulley to calibrate the tach. These methods work but I do't want to mess with the math if I don't have to.

So I am thinking I will call Auto Zone see if they have an inductive timing light with rpm readout, I can borrow for an hour or so. Run the motor at 1,000rpm on the timing light readout, and compare the reading on the timing light to the reading on the boat tach,adjust if necessary, then repeat at 2,000, 3,000, then 4,000.

Does anyone know of a reason this wouldn't work.

Your boat is new enough all you need is a engine computer for Mercruiser.

Plug it in and it should give you real time RPM and sensor readings.

Thanks Little Duck, I have a 4.3TKS any idea where the plug connector is located? Is it an OBDII connector like on a car? If it is will any scan tool work or does it have to be a specific Mercruiser item. If its a Mercruiser item I have no idea where i would get one because I'm sure the dealer won't loan one out, and I would imagine to buy one would cost a small fortune.

The NY Dad said:

Thanks Little Duck, I have a 4.3TKS any idea where the plug connector is located? Is it an OBDII connector like on a car? If it is will any scan tool work or does it have to be a specific Mercruiser item. If its a Mercruiser item I have no idea where i would get one because I'm sure the dealer won't loan one out, and I would imagine to buy one would cost a small fortune.

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Might want to search the forum and see what you can find. Not sure about the OBDII ... I had a 2011 SeaRay for a few years and never seen a port to plug in a computer.

Little ducky I don't mean to be a jerk at all, but you have me confused. First you said my boat, which is a 2010, should be new enough that all I need is an engine computer for mercruiser. What is this mercruiser engine computer is there a real name for it. Then you tell me that your 2011 which is newer than mine didn't have any port to plug a computer in. I looked at my boats wiring diagrams and it doesn't show any place where a computer could be hooked up either. So I guess I'm back with my original idea of using an inductive timing light with rpm function

Your 4.3 is pretty basic and other than the TKS (which is a fancy choke system) is not any different than my 1999 4.3. It's carbureted, has electronic ignition, but that's about it. There is no ECM to plug anything into. Use a timing light or buy a laser tachometer to verify the actual RPM. Agree your tach is off, the rev limit on a 4.3 starts interrupting the ignition at 4950rpm.

I've never looked closely at the tach on my boat, I know there is a setting for the number of cylinders, didn't know there was a calibration adjustment. My tach seems pretty spot on compared to my laser tach.

The NY Dad said:

Little ducky I don't mean to be a jerk at all, but you have me confused.

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Sorry there's no OBDII like a car. If you do a google search for Mercruiser Engine Computer you will see there's a number of adapters and plugs used for various makes models and years costing $$$$. So I bet you have to disconnect some of the wiring harness to plug these things into. I know they had to use a computer on my 2011 a few times to figure out why it was running like crap but I did not witness them using it.

Like Bill mentioned... a carb'd engine, like the OP's, does not have a computer to plug into.

NY Dad... what method do you plan on using to "adjust" the tach?

I plan to borrow an inductive timing light with rpm function from Auto Zone. Put the muffs on and warm the engine up. Run it up to 1000 rpm according to the timing light, adjust the tach if needed, and repeat this process each 1000 rpm up to 4,000. Then Run it up to 4,800 real quick and see if the needle looks like it is around where I think 4800 shoild be on that piece of junk tach. I can’t for the life me understand why they would not have 100 rpm markings on one of the most important instruments on the boat and worse yet why they dont offer one as an acessory.

Please don’t run high rpm on the muffs.

Your garden hose cannot supply enough water.

If I remember correctly, 1300 rpm may be the max recommended rpm.

Thats good to know about the muffs, Thanks fir the heads up, that could have been bad.

I understand the process you're going to use to determine if the tach is correct, or not... but how are you going to actually adjust it?

Theres is a hole in the back of the tach housing labeled Cal. There is a potentiometer inside the housing that you adjust with a screwdriver through that hole. If I had to guess I would say it's probably a 10 turn pot and the gauge is probably set up so 1 full turn equals 1,000 rpm on the tach gauge, but that doesn't really matter. You just turn the adjuster on the pot clockwise to raise the needle reading and counter clockwise to lower it. I would adjust the throttle so the engine reads 1,000 rpm on the timing light then adjust the calibration pot so the tach in the boat matches the timing light reading.

This is in addition to the selector switch for 4, 6 or 8 cylinders? Gotta admit, I never knew that was there! Although, I suppose the reason is because I've never needed to do that. In all of the boats I've owned, run and worked on, I honestly can't remember coming across a tach that was off. Interesting.