New tach gauge is not reading correctly

[shockr]

I have a 1995 Mercury Black Max 150 2 stroke.
I just replaced the tack on the boat, before at WOT tach was reading 5000 to 5200 rpm, the new tack is reading 4200 to 4500. I believe the take is ok because when the motor revs the needle in the tack jumps slightly.
I installed a TELEFLEX PREMIER PRO SERIES model 6272P, the tack is universal and has adjustments for different motors. The manual says to set the dial to 1/2 the number of alternator poles, I have it set to 6 (as I learned that the motor has 12 poles).
Any ideas as why this is happening, what to chase down or what I can try to make it right?
Is there an alternative method to check the rpm at the motor to see if motor and the tach are insink.

 

[Barnacle_Bill]

Re: New tach gauge is not reading correctly

If one of the settings is 6P, it should be set to that. Some gauges have a 6C setting

 

[shockr]

Re: New tach gauge is not reading correctly

Thanks, I have it set to 6P. Is there any other adjustment that might work?

 

[Rowroy]

Re: New tach gauge is not reading correctly

Why did you replace your tach in the first place? How do you know it wasn't reading high?

 

[shockr]

Re: New tach gauge is not reading correctly

I replaced the old tach, for a couple of reasons: 1 it fogs up bad in the mornings, 2 the light socket on the gauge was fallling apart and was not just the bulb, 3 the needle would stick / or would be slow moving to slower or higher rpms, 4 at WOT with the 3 blade prop I am using right now the motor will chirp and the gauge did not show it.

You are right I really dont know. The only thing I can do is go by sound and the motor is up there, and the with dealing with the prop guy I am using, the motor - boat- prop - gives me a speed that is close to the formula he uses which I believe that old gauge at 5200 was correct andthe new gauge sits at 4300 or there abouts.