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Discussion Starter · #1 ·
I'm using an external tach during engine break in because the gauge cluster is out of the car.

If I connect the tach directly to the tach terminal on the HEI, it reads just fine. But, if I instead connect the tach through the tach filter it reads fine at idle and up to about 1200 RPM. Once I rev the engine above 1200 RPM the tach starts to swing back and forth from about 500 to 1800 RPM.

Is the tach bad or the filter? Any ideas?

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