Tach showing wrong rpms

[FrenchFry]

The engine is a Yamaha 225 HP 2 stroke 1990.

A strange thing occured last time I used it, on the way back the tach was showing about twice the rpms, usually I go 3500 rpms, the engine can go up to 6000, and the tach was showing 6000/7000 rpms, then slowing down before arriving it was showing 1400 rpms instead of 700/800, but clearly the sound of the engine was not 1400 rpms but 700/800 and the engine seemed to work fine all the way back.

Checked the prop, cps, cables to the tachmeter and did not notice anything, restarted the engine one day later and it indicates the correct rpms.

Maybe that's just the tachmeter that is indicating bulls (or the cps getting crazy but as far as I know it's rare) but I would like to make sure that a major pb is not coming, any idea why it did this and what to check if it happens again? That's the first time it's happening.

 

[Jackson 88]

Every so often this will happen with my Yamaha 4 stroke. Seems after the motor is shut off and restarted the tach works normally. I have a friend who also runs a Yamaha and he says it happens to him also.

 

[gm280]

Is your tach an after market type or di it come with the boat setup and is factory? Reason I ask is because if it is an aftermarket tach, there is a selector type switch on the back of the tach that can be set for different type engines. So some times you can switch that back and forth and clean the switch setting for a more positive reading. It happens on here quite a bit... JMHO!

 

[robert graham]

Is your tach an after market type or di it come with the boat setup and is factory? Reason I ask is because if it is an aftermarket tach, there is a selector type switch on the back of the tach that can be set for different type engines. So some times you can switch that back and forth and clean the switch setting for a more positive reading. It happens on here quite a bit... JMHO!

Yep, just switch that little slot head screw on the back of the tachometer back and forth until the RPM's are obviously correct....seems those little contacts on the inside of the tachometer get dirty and the movement of the switch sorta cleans them...

 

[FrenchFry]

Thanks for the answers, sorry for the late reply, the tach is a Yam original one that I installed recently (but still with a selector) since the former one decided not to display anything any longer (I have almost everything in double on that engine), but the thing spent some years laying on a table without me taking care of it (maybe the contacts inside are rusty/dirty).

The selector is on the right position, I will try to move it as you suggest to reset/clean it, as far as I understand that's not a critical problem, it happened yesterday again, strangeley the sequence is always: way in, everything ok, correct rpms, bad sea conditions (console/tach getting wet), way back after some hours with the engine off, leaving the place at low rpms during some time, correct rpms until accelerating on bad sea conditions --> wrong rpms, tested with two different props and same behavior, the issue does apparently not show up in good sea conditions.

I will follow up.

 

[robert graham]

My tachometer started that inconsistent business whenever I hit a wake or some chop in the water....turned out to be a slight bit of corrosion on the in-line fuse....maybe check, clean and grease your fuses....that humid marine environment does things to electricals....

 

[FrenchFry]

Thanks, the behavior seems to be the same, but not sure to understand what is an "in-line" fuse, you mean the fuse outside of the tach connected to it?