New tach is waaay out`

[gitz676]

I bought a new in the box tachometer from a surplus store recently. After hooking it up to my HEI dizzy I was surprised to have it read 4000 RPM at idle. When I rev the motor up it jumps up to 8000 RPM. What do I need to do to calibrate it??

 

[SrBeaver]

it should have settings for 4cyl, 6cyl and 8cyl. Look in the instructions or google it and it will be able to tell exactly how to change it to match what you're running it on.

 

[54cj3b]

there s a switch on the back

 

[gitz676]

there s a switch on the back

No switch or instructions. Will try Google again. Thanks anyway.

 

[BusaDave9]

If there is no switch to calibrate it for a 4, 6, or 8 cylinder then I think you have a tach for a 4 or 6 cylinder. It can't be used on a V8.
Tachs are not one size fits all. If it says it is for an 8 cylinder then double check how it is connected to your coil.

 

[Old Dog]

Do you have a MD system?
Theres always the chance that the tach is junk.

 

[gitz676]

Do you have a MD system?

What is a MD system. Searched Google and this forum to no avail.

 

[BusaDave9]

What is a MD system. Searched Google and this forum to no avail.

Not sure but maybe Old Dog meant an MSD ignition
If your tach has no adjustment button it must be for an engine with a certain number of cylinders. If you have an 8 cylinder and a tach for a 4 banger it will read double the correct RPMs. If it's a for a 6 cylinder then it will read 50% higher than it should.

 

[Old Dog]

Not sure but maybe Old Dog meant an MSD ignition

Yep thats what I meant, I need to edit better.
somehow I forgot about this thread.

 

[gitz676]

I have an HEI system with two connections, Tach and +. I think B-D-9 is right and the tach is for a four cylinder. Maybe if I put two in parallel I will get the correct RPM. lol

 

[Buuushman]

On the tach for my truck the switch was inside the housing- might take a peek and get lucky.
Bush