Factory Formula S Tachometer question

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lou3500

lou3500

Fla. Cuda Guy
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I have a 66 Barracuda 273/auto with a Fourmula S Factory tach that was rebuilt still in the post office box when i bought the car it came with the wiring diagram hand written by the re-builder . I wired it per the diagram and it works however the needle floats at throttle it's not jerky moves it just swings 500-800 rpms . The in cluster gauge looks correct but the circuit board is mounted in a square plastic box i have under the dash not like the round type in the kick panel , What could be the issue . Thanks for your time. I have a video but even after trimming it down it's still too large.
 
Stock points ignition or some aftermarket spark box? Tach input may need a diode inline. 1N4004.. stripped band towards the tach.
 
Stock distributor with Pertronics upgrade [no points, ballast resistor removed] stock type round coil MSD Blaster Red type.
 
Might be varying battery (alternator) voltage. Put a voltmeter as close as you can to the tach power supply terminal and see if it matches the "swing."

The box you describe indicates some form of aftermarket tach circuit. It may or may not be a good design.

Unfortunately it could be the meter itself Some of those (like the marine ones) had very "floppy" meter movements.

Also, see if the tach meter head is suffering from static electricity. To see that, turn off the key, and gently run your finger across areas of the face. Static will change as you do so and the needle will move. There are various "anti static" chemicals you can find
 
The voltage regulator was swapped to a Solid state unit , this gauge has done this since i installed it 15 years ago my OCD has over ridden my pretending not to see it any more .
 
I think I answered this on a Facebook post. Try Real Time Engineering (RTE) for their tach sender part.
 
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