Might need some pictures or more information. Most tachs that I have seen have switches on the back/inside to switch the number of cylinders being monitored.
From Autometer's website:
What Are These Colored Wires On My Tach For?
• Red = key on power supply
• Black = common chassis ground
• White = dash lighting power
• Yellow = used only on Internal Coil Magneto tachometers as the signal wire
• Twin Black = (looks like a lamp cord) is used on External Coil Magneto tachometers that use an inductive pick up (#5210)
• Green = tachometer signal for standard or electronic ignitions
• Gray = for tachometers that use a Hi/Lo shift light. When grounded, Lo shift light point is active. When ungrounded, Hi shift light point is active.
• Blue = used on Playback tachometers to start recording (see playback tachometer instructions)
And/or
My Tach Registers/Shows the Wrong RPM?
The causes of this type of problem can vary. We recommend that you check the following before contacting our Tech Department:
1.) Our tachometers come pre-set from the factory as 8 cylinder. Has the tachometer been properly adjusted/calibrated for your application?
2.) An insufficient power or ground will cause a tachometer to actually read too high. If you are experiencing this problem, perform the following:
Using a voltmeter (not a test light), measure across the Red wire and Black wire coming from the tachometer while the engine is running. DO NOT USE AN ALTERNATE GROUND! By doing this, you will be looking at what the tachometer sees for power and ground, combined. If you measure below 12.0 volts, identify which connection is the problem, and correct it.
3.) If you find that your tachometer is off by a very small amount (such as 2 to 400 rpm), consider what you are comparing it to. Many factory tachometers are slow to respond due to heavy dampening used for smoothing pointer operation.
4.) If you are using something other that a 4, 6, or 8 cylinder application then make the tachometer you chose is correct for your application.
If after checking these items, you feel that your Auto Meter tachometer is still off calibration, please visit the Contact Us or Ask A Question pages to inquire about how you may send it in to us for service.
