5 replies to this topic

[Zami]

Tach started reading low at speed. 900 at idle is ok. But at different speeds, 60 and 70 mph it reads low by about 500 rpm per guesswork chart. The spread gets larger above 70. RV1 converted for elect. Ignition. Checked all connections seem ok. Running 3.44 gears. I keep thinking ground wire. Any thoughts?

[Shooter63]

Are you sure the tacho is reading slow? You can check it with a decent timing light that has rpm, also check your speedo reading against a sat nav or phone app. I don't know how old your tacho is but the early Smiths kit wasn't that spectacular.

Shooter

[Spider]

Check the calibration of the Tacho. One easy way is to get a hold of a 12V AC Plugpack. Power the Tacho up off 12V DC as per normal, and the connect the Plugpack between the Tacho -ve and the Points Input to it.

 

If the Mains Frequency where you live is 50 Hz, the Tach should read 3000 RPM, if it's 60 Hz, it should read 3600 RPM.

If you find it off, there is a trim pot inside you can adjust it with.

[DeadSquare]

Please don't take this as unkind or sarcastic.

 

It would be nice if it read accurately, but if you aren't using a full-race cam, where gear changes could be critical, a tacho is just a fun addition.

[Swift_General]

Your tacho may well be inaccurate. But I wouldn't assume so based on expected RPM vs road speed. How do you know speedo the speedo itself is accurate or that the gearbox has the correct speedo drive installed? I think the first think I would do is validate your tacho readings with another source. Some timing lights display RPM for example.

[Zami]

Thanks for input.

 

tach had been working fine .  It just started reading low.  I did check speedo against an app that uses gps for speed calculation.  So I am pretty sure of the tach readings at 60,70 and 80 mph.  I may dig up a spare tach and use it to verify if I have an issue.