portable tachometer settings

[porta-homer]

I had my porta-bote out yesterday, with my new 5hp 4-stroke nissan. I borrowed the local dealer's hand held tachometer to test the rpm's because this new motor has not been pushing my boat nearly as fast as porta-bote said it would. My question has to do with what to set the tachometer to. The dealer said to set it to 2-stroke, so I did, and it showed 2500 rpm at full throtle with just me in the boat. When I set it to 4-stroke it showed 4500 rpm at full throtle. Now my dealer isn't sure what rpm it should have been set at to get a true reading. Does anybody have experience with these tachometers?

 

[Salmon Tub]

Re: portable tachometer settings

How does the tach hook up to the motor? I was under the impression that most portable tachs were the kind that you put a sticker on the flywheel and it picks up the RPM via IR or laser. If that is the case, there should be no need for any settings. Otherwise, as I understand it, you need to have a rectifier installed on the motor in order to get a reading.