I just bought a tach and just wanted to hook it up to check idle speed and set timing. I hooked the black to ground, the red to positive on the battery and the green to - on the coil. I made sure its set for 8 cylinders. When I start the car, its not working. Anybody have any idea what is going on?

 

Does the needle move at all?

You are right on the wiring.
What kind of Tach?
Are the DIP switches definitely set at their limits for the 8 cyl setting?

 

Maybe it isn't getting a good enough ground. Your wiring is definitely correct though.

 

Isn't there a fourth wire? It should be for a light and not matter but...

 

i havent hooked up the white wire for the light yet and it still works fine. check all of the connections again, maybe scuff them clean

 

I did not hook up the white wire because it is just for the light. the needle does not move at all. I will try making better connections.

 

make sure you have it connected to the ground on the coil as well, if you are unsure try both and see if one works

 

Is this one of those mini-tachs? I had an 8K mini tach. When idling, the needle didn't move. My Mustang had to be above 1000 RPM before I saw the tach needle move.

 

After rechecking connections countless times I exchanged the tach for a different one of the same model. Turns out the first was defective. Now I am trying to get the rpms down to 500 at idle. What do I need to adjust on the carb to lower the rpms?