Hood mounted tach inaccuracy

[67FBGT]

Some years ago I purchased one of those repro hood tachs, a repro of those available after-market from way back around '69 or so. Subsequently I think the manufacturer has gone out of business.
It looks the goods, but reads high, consistently I would say around 50% high compared to the rev readings of both my  engine analysis tach & an in-dash tach which at least both agree.
Has anyone got any ideas as to why this might be & how I could correct it so it at least reads closer to reality? Am wondering if an in-line resister might be what's needed. Many thanks, John

[booah]

Maybe the selector switch is set to a 4 cal, doubling the rpm output?

[StephenSLR]

Maybe the selector switch is set to a 4 cal, doubling the rpm output?

If it works proportionately like you suggest, then if it's reading 50% high it'd be set to 6 cyl.?

Check to see if there's a switch for the no. of cylinders and set accordingly.

s

[bluestang]

I put a hood mounted Tacho on and there was a switch to change it from 6 to 8 cylinder. I can not remember if the switch was in the guts and only accessed by taking it apart or accessed through the bottom of the casing. Mine reads true.

[bluestang]

I put a hood mounted Tacho on and there was a switch to change it from 6 to 8 cylinder. I can not remember if the switch was in the guts and only accessed by taking it apart or accessed through the bottom of the casing. Mine reads true.

[67FBGT]

Thanks guys, there was a 3-way switch underneath that was mis-labelled but after a little experimentation the problem was solved. Cheers.

[mussy65]

Yet another correct diagnosis with corrective action provided ... in 90 minutes!

Well done brains trust.

[IGALOP]

That's us, always on the ball and ready to remedy!:smile01:

[j1autotech]

That's us, always on the ball and ready to remedy!:smile01:

Hmmm... how's the rust situation going on my car ? isn't it time for another wash