tachometer worked last year now way off

[nailem]

I have a 1986 mercruiser 5.0L. i bought the boat last year and verified the tach was correct using a digital tach. this year i put the boat in the water and the tach is way off. at idle boat tach reads 1200rpm digital meter reads 800rpm. at WOT boat reads 6000rpm meter reads 4300rpm.

why would it be off after sitting. im going to double check timing (even though i did not change it) but what else could it be?

is my method for checking the rpms wrong? i am using a digital tach used in HVAC to check fan and motor speed. it uses reflective tape which i put on the harmonic balancer and the meter sends out a beam bounces off the tape displays the rpm.

 

[JoLin]

Re: tachometer worked last year now way off

Try something simple that worked for me- behind the tach is the little switch that you use to set the number of poles for the motor you use. Turn it from side to side a couple times, then back to the proper setting.

I found that tip in an old forum post last Spring and it straightened mine right out. A little corrosion buildup was the explanation, I think.

 

[nailem]

Re: tachometer worked last year now way off

thanks i'll give it a try