Tach Calibrate

[italianstal27]

Hi friends,

So experienced a weird anomaly today... We finally got the engine running (huge thanks to everyone here!), and it's in the boat. Set our Faria tach to the #1 position correlating to a 4 cylinder engine, and it's reading ~2300RPM.

thought that's weird. Borrowed my friend's photo tach, measured 750RPM off the timing light (not the most accurate I know). Then, went to the harmonic balancer (doesn't exist on a merc 120, but not sure what to call it), and measured it at 1500 RPM. So those allign near perfectly.

So on the tach, I now have it in the #4 position, correlating to a 10 cylinder engine according to the manual.

Anyone else ever found the tach isn't right according to a timing light?

Also 750 RPM with no cover on seems pretty fast...

Thanks,
-John

 

[Boater31]

Last year I had a tach that acted like this. As soon as the engine started it would read between 1500-2000. It would act normal just offset 1500 ish revs. I knew by the engine sound it was at idle. With the advice I got here I moved the cylinder selector between each selector to maybe clean the contacts. I didn't use any spray or anything just quick movements. After selecting my cylinder count it has wired fine ever since.

 

[thunder550]

I've read a lot about Faria gauges and loose or corroded grounds causing bad readings. When it reads high, knock on the gauge face a couple times and see if it settles in where it's supposed to be. I had this problem with my new-to-me 1995 Cobalt. The tach would occasionally read 2000+ RPM when I was idling, but I could knock on the gauge face twice and it would correct it. Finally got behind the gauge and tightened the grounding nuts and it's been reading correctly since.

 

[italianstal27]

Grounds are secured properly, gauges are brand new. Did the full sweep on the tach, and it's still reading 2000RPM at idle.

Anyone think of any problems running the tach with the 10 cylinder setting?

-John

 

[Boater31]

I can't comment on the keeping it set too 10. However when the expert's get here they will want a SN. I'm not sure about newer styles but on my boat the tach just reads pulses from gray wire on the coil. Have you checked the connection there if it applies.

 

[italianstal27]

Yeah coming off the grey wire... It's a Merc 140 1973 service manual #2...

Used the photo tach on the timing light for all 4 cylinders. All are around 750 rpm. Cylinder #2 is doing 1000.

-John

 

[wellcraft-classic210]

Hi
If your confident in all your connections and wires then it would seem the tach internals may be damaged or defective. Their are some circuits inside the tach that feed a coil and create a magnetic force to rotate the needle -- the engine cylinder count switch is also part of those ckts.

It seems you may want to try another tach using those same connections and see how it behaves --

If it solves the problem then you will know the existing tach has problems which are not easily resolved such as a cold or open solder joint or defective transistors etc. ( unless it turns out to be connections or switch settings )