Factory tachometer reading high

[Kcorey]

Hey guys , I’ve got a 72 k20 with factory tachometer . I rebuild the engine last winter after buying truck , and ever since I installed it my factory tachometer reads roughly 3-400 rpm high at idle . And when I rev the truck up or accelerate , it seems to respond rather quickly ( meaning rpms show they are rising but definitely not matching the actual engine rpms). Just looking for some thoughts , it’s a 350 with a MSD hei distributor . Thanks

[HO455]

Mine is the same way. I believe the 50 plus year electronics have degraded and are causing the erroneous readings. I know that member Tbone1964 can fix the problem.

[Sheepdip]

Originally Posted by HO455

Mine is the same way. I believe the 50 plus year electronics have degraded and are causing the erroneous readings. I know that member Tbone1964 can fix the problem.

I'll second TBone, he does good work.
And while your at it and got the dash out, have him fix you up with a volt meter to replace the worthless amp meter.

[custom10nut]

Two possibilities here.
As mentioned, the 50 year old electronics are probably worn out, but they will usually read way too low (mine read a consistent 225 when the actual rpm was 750).
The fix is a new tach or a rebuild, which will cost you about three times as much.
The other is that it needs to be adjusted, by turning the screw in the center of the back of the tach.
If it just started doing wacky stuff out of the blue? It’s probably the Electronic issue.

[Kcorey]

I can’t say if it just started doing it , because when I bought the truck there was no motor in it . Since I put the new engine in, it has been doing this. I’ll look at the adjustment

Originally Posted by custom10nut

Two possibilities here.
As mentioned, the 50 year old electronics are probably worn out, but they will usually read way too low (mine read a consistent 225 when the actual rpm was 750).
The fix is a new tach or a rebuild, which will cost you about three times as much.
The other is that it needs to be adjusted, by turning the screw in the center of the back of the tach.
If it just started doing wacky stuff out of the blue? It’s probably the Electronic issue.

[custom10nut]

I think there’s an adapter you have to use with the HEI, to get it to read correctly.
I hope someone on here can give you better guidance

[kidatheart]

Is it possible it’s a 6 cylinder tach? How would it read if hooked up to a 8 cylinder?

[Andy4639]

Checkout this post for a cheap way to fix your tach. It's less than $70.00 to fix it and it will work with HEI and read correctly. Either choice is good.
I just bought both and put the new one in the Ramp Truck. The rebuilt one will go in a cluster to sell maybe are keep as a backup.


http://67-72chevytrucks.com/vboard/s...23#post8706423

https://www.youtube.com/watch?v=epOYrxJV-a0

You Tube

[gmc684x4]

Check this thread out a new board for your tach if it needs rebuild

http://67-72chevytrucks.com/vboard/s...d.php?t=803315

[Andy4639]

You can pull the tach out of the cluster and hook it back up and run a ground jumper to it. Crank the truck up and set the tach per how it is running using a dwell meter if you have one that has the tach feature. If it's a factory tach it will be stamped on the back of the tin where the wires hookup either 8cyl or 6 cyl. All the ones I have ever owned did anyway.