connecting tach/ determining RPM

[brunolund]

I have a 2005 mercruiser 50 4 stroke efi. I am trying to find the correct prop for my pontoon, which does not have a tach. I tried using my multimeter on the tach setting, on the grey wire. that doesn’t seem to work. I also tried two different tachs from amazon, again using the grey wire. the numbers are of by over double, even when the tach is set for a 4 cylinder. I also tried using my timing light, that has a tach function, with the inductive pickup on cyl 1 spark plug wire. idle numbers looked good, but then as soon as i added throttle, the numbers were double what i’d expect. if i put the the timing light in the 2 stroke setting, idle was off(450), but the mid and high numbers looked and sounded correct(4800). I think i still need to go down in pitch, but want to make sure i’m reading the Rpm correctly. this engine I believe has cdi ignition, with a separate coil per cylinder, so i can’t see it having a waste spark system. any suggestions or thoughts? i’d love to hear them. thanks.

 

[brunolund]

the two tachs i tried were both automotive tachs, if that makes a difference.

 

[Chris1956]

The tach likely runs off stator pulses. They do this because every HP motor has the same number of stator pulses at a given RPM, so a universal outboard tach may be used.

I would have thought the timing light tach would have worked. The automotive tach's are probably hit and miss. That is, they might work.

Why not buy an outboard tach, made for your motor? You will need ign power, ground and the grey wire signal to make it function. Set it on the specified pole or cylinder number and test it.

 

[Scott Danforth]

get a tiny tach (or the china knockoff from amazon )if you want something cheap.

or get a proper outboard tach for the outboard motor.

 

[brunolund]

I would have thought the timing light would have worked also. my boat doesn’t have a spot for a tach, and wasn’t planning on installing one. I suppose i could, but figured once i get the correct prop i don’t really care. My buddy wants to mount one on his boat, so that’s why i had the cheap chinese ones to try, but obviously they don’t work on boats. after doing some reading on the internet, I understand the stator pulse count for outboards. I believe my motor has a 12 pole stator, so i,d set my a marine tach at 6 pulses? is that correct?

 

[Scott Danforth]

Get a tiny tach. It has one wire you wrap around a spark plug. Use it as a tool to check your RPMs.

Then when your done, put it on your lawnmower

Or get an optical laser tach and read the flywheel