Tach accuracy?

[J Lamb]

Swapped my analog Yamaha tach out last winter for a digital Yamaha tach. Noticed I had lost a couple hundred rpms but I just attributed it to heat and humidity loss on a carb motor(150 ProV). This thing use to turn 5700 rpms in summer but I could barely get 5500. Went fishing last week and took my Fluke meter and checked it on min/max setting. Tach read 5700 and max reading on my fluke was 5917 rpms. I always kinda figured this tach was a little low now I'm sure, I think. Should I trust the meter reading and is there a way to calibrate a Yamaha tach?

[Bill Rogers]

Trust the meter - a fluke meter set to read frequency is a very accurate way to measure rpm and check the tach. You shouldn't need the min-max function though. Just set it to read freq. The conversion is 10 rpm/hz for 12-pole alternators. At 5917 rpm the fluke will read 591.7 hz

Don't know about calibrating a Yamaha tach. Some tachs have an adjustment that's accessible through a small hole in the back.

[J Lamb]

Should've explained better. Have an inductive pickup for my 78, makes it easy. Set to min/max rpm and left it under the cowl hooked to #1. Cord isn't quite long enough to hold and drive. Any way I kinda figured I could trust the meter more. Now I know I'm 200 rpms off. I feel a little better. Now I know the analog was more accurate than the digital.

[David]

How do I calibrate my Merc tach? The gage in my SS2000 reads low.