2018 25 Mercury EFI Tach Readings/Settings

[MartyLowe1546]

Good afternoon everyone and in advance I appreciate everyone's input. I recently purchased a Lowe 1546 jon boat paired up with a 2018 25 mercury efi four stroke. Shortly after I purchased a tach off amazon to determine what pitch prop to purchase because with the current 12.5 pitch prop it didn't sound like it was turning the rpm it should. My question is the tach I purchased allows me to set the sparks per revolution. From what I understand I set it at two sparks per revolution. The signal wire is wrapped around the top cylinder plug wire in the middle of the wire between the boot and coil and I have the excess wire coiled up and tucked under the cowling. The rpm at idle seems some what accurate and WOT is where I wonder if I am over pitched or I have an inaccurate reading which the highest I've seen it read at WOT is 3900rpm. The mercury manual says 6000rpm at WOT. Any input or advice on setting this tach up is greatly appreciated. The tach I purchased off amazon was the Foundown engine hour meter inductive tach.


Thanks

Marty.

 

[muddywaders]

An inductive tach should have two settings: one spark per revolution(2-stroke) or one spark per two revolutions(4-stroke).If your tach is set for the former you would be at 7800 rpm.Something is amiss.I have purchased a tiny tach and although it was useful for propping(WOT rpm) I found it erratic below 1000 rpm it also stopped working very shortly.I now use an automotive style voltmeter for tuning purposes.I believe I may have bought a faulty unit because other people have had good service from their "tiny tach".Hopefully someone can recommend a particular brand/model.cheers m.w.

 

[MartyLowe1546]

I really appreciate you taking the time to write a reply. If I am understanding you correctly I should set it to 1 spark for every two revolutions ? I tried setting it to 1 spark per every revolution and it was extremely high which I knew was incorrect.