How to Halve Pulse Frequency From Outboard Engine for Accurate Rev Counter Reading

I am currently working on a project where I need to measure the rev count on an outboard engine. I have a rev counter but it currently shows exactly twice as many revs as my calibrator. I'm thinking it has something to do with the engine sending out 12 impulses per rev. The question is: is there any way to half the number of impulses with some kind of frequency regulator etc.?
I hope that anyone can guide me in the right direction. 
Kind regards

How are you physically measuring the revs from the motor, off the drive shaft or propeller, for example. How many cylinders, the rev info may come from the plug firing, for example, not something I know much about.If your measurement setup is correct, and you are sure that you need to divide by two, you could use a flip flop, you will likely have to condition your input signal, it would need to have  appropriate voltage levels, rise and fall time, and the signal must be clean, that is, no glitches or  analogue content. This may necessitate using any or all of the following, filters, limiters  and a schmitt trigger. A scope might be needed to ensure that these conditions are met.Cheers,Richard

Thank you for the answer, Richard!
Well I am physically "calibrating" and thereby measuring the revs with an optical sensor and a tiny reflective sticker that I mount on the top of the engine. The results I get from this are pretty consistent but it is not possible to do a measurement while sailing, since there is too much movement.There is though an outlet on the engine itself meant for a rev counter. But my rev counter only officially supports 4-cylinder engines, while my testing engine is a 2-cylinder. I have never thought about using a flip-flop for the project. To be honest I have no idea which kind to use and in what ways it might alter the signal. I had the impression that I could use some kind of regulator like the ones controlling the backlight flicker on TV's. Or am I on a wrong track here?
- Lasse

Not sure where you are coming from re the backlight flicker is concerned. A flip flop, being a digital device, requires a clean signal, the precise specifications depend on the type of logic family being used. Standard slow speed CMOS , the CD series available from many manufacturers has a supply requirement of 3-18V,  you probably have a 12V supply available for the motor if it is an electric start. A D FF is perhaps the easiest of the standard packaged FFs to use, you typically get a number in a DIL package. As far as altering the signal is concerned, you may have to do that in order to make the FF work, but the circuit will not load the source if designed correctly. I am assuming that the rev counter output is purely digital.Since the rev counter is dependent on the number of cylinders, then my guess that the signal comes from the circuitry associated with firing the plugs is I assume correct.
As I said, it would help if you could describe the waveform you are getting out, and the best way to do that is to use an oscilloscope.The other side of the picture is that you would need to know what sort of signal your existing rev counter needs to function, if it is designed to plug into marine outboard engines the specs may not be available if it is propriety.It may be easier to set your engine mounted  optical sensor up so that it would work at sea, rather than to try and adapt your existing rev counter, or quite frankly buy a rev counter that would work with the engine. Hard to know without a lot more detail and a few pictures, cheers,Richard

What does the output of the rev counter feed? Is it just a display or is it going to some sort of control or monitoring system?If you know that the output count is exactly twice the number of revs, you could just divide by 2 when processing the data...If the rev counter is designed for 4 cylinder and you are using it on a 2 cylinder, I'd expect it to register half the revs not twice if it's just counting spar plug impulses. On the other hand,  years ago I remember automotive rev counters that just hooked to one spark plug so it didn't matter how many cylinders your engine had. It may be a 2-stroke vs 4-stroke issue.