Tachometer calibration

[425HP409]

I have three tachs and none of them read accurately when hooking them up to a running car with a known good tach connected to the engine. One is slightly high and the other 2 are about 20% fast. Is the small screw on the top rear of the tach a calibrations adjustment?

[Palf70Step]

The one in mine has never worked accurately since the HEI was installed. All the tricks have been tried, but it never worked accurately. Not sure why, but HEI doesn't give the right pulsing or something like that. I don't remember any of the adjustments,if there were any.

[chewychevy67]

This is why I got a 4k diesel mechanical tach and had member tbone1964 recalibrate it to 5k and he also rescreened the face to a 5k as well. He did a great job. Also had to get a distributor with the tach drive as well. I have not used this yet as my truck isn't complete but I'm hoping it will be more accurate than the electronic tach.

[custom10nut]

To hear the frustration some go through with the factory tachs.
I’ve not had any issues with either one of mine.
For many years I had a used one from a 72 Blazer, and never had an issue with it, even when I put a Mallory Electronic Conversion kit in the Distributor, to replace the points, until one day the Diode in it “Gave up the Ghost”.
I replaced it with a new factory model, and it works great.
Gauging it against my Digital Multi-Meter Tach it’s within 20 RPM, so I’m not messing with something that works.
I must be fortunate, because I don’t have any of the mentioned isssue.
Good luck on figuring out the problem.
And about your adjust me t question.
Yes the screw on the back IS the adjustment screw for “Fine Tuning” the Tach.
It helps to have someone helping you, because you have to be laying on your floorboard while you adjust the screw. Good Luck

[425HP409]

I found a previous thread that addresses calibrating it with an adjusting screw that is at the top rear of the tach that is covered by tape.

[leftybass209]

I can add that, I have a tach that was reading low, also HEI. I assumed the board was bad and replaced it, using a new board from Cajun Tach shop. It still reads exactly the same, and has a hangup where it will freeze.

I assume the issue is with the physical mechanism needing recalibration, not the electronics.

[425HP409]

I hooked up 3 tachs to a car with a digital timing light for a reference tach and 2 calibrated perfectly throughout the rpm range with the screw in the back. The 3rd would calibrate at idle, but read 2000 when it should have been 2500. I have 2 more to check tomorrow. ps: The test car had HEI and one of the tachs had previously been in my truck with point ignition. It read the same before calibration with both ignition sources. The tack reads ignition pulses, and doesn't care if it is HEI or Points.

[CSTGURU]

I just thought i would provide my findings i have with 2 used original 8cyl tachs. i am running a v8 with original 70's era GM HEI distributors. the tachs work fine and read pretty accurate(maybe out by 100 or so) when comparing to my multimeter. like 425HP409 has stated, it seems they operate off pulses from the distributor, so as long as my HEI does not wreck any of the original Tachometer circuitry, im happy to use them!

I do however have a 3rd original tach that seems to want to read over 500 RPM higher in all ranges, so im thinking its a problem with the tachometer or an adjustment, (the zero adjustment is good tho)

[badpeanut]

I have a factory tach (not reproduction) and HEI. I temporarily installed an aftermarket tach and adjusted the factory tach to match. Without AC, I was able to reach up behind the cluster and adjust the screw on the back of the tach; it doesn't take much to make a big adjustment.

[Already Gone]

Originally Posted by leftybass209

I can add that, I have a tach that was reading low, also HEI. I assumed the board was bad and replaced it, using a new board from Cajun Tach shop. It still reads exactly the same, and has a hangup where it will freeze.

I assume the issue is with the physical mechanism needing recalibration, not the electronics.

Oh goody. I have one of Cajun Tach Shops new Circuit Boards for my tach and was hoing was going to sure my not reading accuaratly problem . I am using a Petronix stock look distributor.