Basically over old tach's that fail due to 50yr old components,certainly had my share of issues with them ,the wise mans fix is a replacement of the driver board ,have worked with Bob @ ashlocktech for a long time first by buying his tach adapt then he assisted me with the best "bits" for my 911 ignition and my current work with tacho driver board .
For you 4 cyl guys its relatively cheap to buy a replacement and you will be sorted but 6 cyl tach's are becoming more expensive in working condition for example a silver base needle will still need bezel removal and interior clean due to the way they mount in a 911 which is open in the trunk.
My solution was to purchase Bobs driver board and get a company in my country to delete the old board for the new one , calibrate and reset needle damper and ready to go for a modest price,really happy.
First pic is old driver board and next with new one fitted.
Click to view attachment
Click to view attachment
Bob has been really great thru my processes ,highly recommended,will update as other work on the go.
Cheers.

I believe this is similar to how places like Palo Alto Speedometer fix our old tachs these days. They completely replace the guts with modern electronic mechanisms, rather than figuring out what component(s) have failed and replacing them.

They did mine a number of years ago, and mistakenly set it up as a six-cylinder tach. Which caused it to read very, very low because it was expecting six ignition pulses per engine revolution and it was only seeing four... Fortunately I live nearby and just handed it back to them, and was able to come back the next day to get the re-fixed one.

--DD

Tacho now back together again as I fitted an led light strip ,I have these in 2 of my other main gauges and like how they provide light to the edge of the gauge face where you need it most and of course the brightness is much improved over stock lamps or single led ones.
Not shown but added an earth tab near the exit of the led strip wires, this is neater and effective as earthed to tach housing and you only have one wire to connect to the lighting curcuit to make them work,rear of the gauge body the led wires are hot melt glued in so they are secure ,this tach required one original lamp diffuser removed to feed wires.
Click to view attachment.
Click to view attachment.
Cheers and more to follow .

Well my happiness at finding someone to repair my tach was short lived,the tech doing the work sorta flaked out on me ,it won't work and just barely responds to cranking with a small needle bounce.
Suspected input rate was off and questioned him,his reply was he had adjusted the pot on the new board to 4 cyl engine ,I answered his early question about cyl count before he shipped it back to me .
Strange how he has gone to ground on this considering I have paid for the work .
I guess time will tell and I'am patient enough but day 3 and no reply ?.
Will update when or if I get solution,cheers

Update the tech sent me a reply saying he had set up for 4 cyl rate as mentioned and then back pedaled saying he had marked 6cyl on his work sheet,strange guy communication wise pretty much didn't read bugger all I emailed him about .
End result is a very steady responsive unit and responds as it should to idle and full rpm on limiter rotor.
Had this pic and it shows gauge earth to body for an early tacho before led strip install,cheers.
Click to view attachment

This is a reference thread for tachometer upgrade:

http://www.914world.com/bbs2/index.php?sho...216719&st=0