Auto tach use with 2 stroke 3 cyl outboard

[kenimpzoom]

I need to adjust my idle and I have an old shop tach for a car. It has a 8 cyl and 6 cyl scale.

My motor is a 2 stroke 3 cyl Mercury sport jet 90 (yes, I know its a Force motor). The tach output comes from the voltage regulator.

I have hooked up my shop tach to the tach output and it gives a reading.

My question is, should the tach output be directly read on the tach 6 cyl scale, or is there a multiplying factor for the 2 stroke?

Thanks,

Ken

 

[Frank Acampora]

Re: Auto tach use with 2 stroke 3 cyl outboard

Your tach is counting pulses from the 20 pole alternator. Force engines (and some Mercurys) use a special tach. I don't know if there is a known factor to correct your tach. It's not the number of cylinders that counts here, it's the pulses from the alternator. And, since there are discrete windings for ignition and charging circuits, I am not sure if the full 20 poles are used in the charging circuit so not sure how many pulses you are really counting. But I am reasonably sure both 6 and 8 cyl scales will be wrong.

 

[kenimpzoom]

Re: Auto tach use with 2 stroke 3 cyl outboard

So if I wanted to add a marine tach to my boat, what should I use? Would it require a special Mercury tach, or will any outboard tach do.

FYI, I have the "red stator" in this motor.

Thanks, Ken

 

[Chris1956]

Re: Auto tach use with 2 stroke 3 cyl outboard

Ken, There are a line of universal outboard tachs that work with Johnnyrudes and Mercurys. You need to check to see if your Merc (Force) is covered by these. If you motor has a distributor and a single coil (not very likely), you can try the auto tach, connecting it to the plus side of the coil, using the 6 cylinder reading, for tune up purposes.

 

[kenimpzoom]

Re: Auto tach use with 2 stroke 3 cyl outboard

I am gonna put it in the water soon and see what it reads for idle connecting to the tach wire.

Chris, thanks for the idea on the coil, it is 1 coil/cyl, but I should be able to multiply the readings on the 6 cyl scale by three.

6 cyl 4 stroke single coil = 3 pulses/revolution
3 cyl 2 stroke 1 coil per cyl = 1 pulse/revolution

Is my math right?

Ken

 

[Chris1956]

Re: Auto tach use with 2 stroke 3 cyl outboard

Ken, If you have one coil per cylinder, you can only count that spark, then the tach will read 1/3 of its RPM. Since you are using an automotive tach, and it is a 2-cycle motor, multiply the reading by 1.5 to obtain the aprox RPM, using the 6 cylinder (4 cycle motor)scale.