....well, to me at least.
2 weeks ago on a drive my tach started freaking out, bouncing all over the place. This is an original tach restored by Phil Severance so wasn't overly concerned about it being the tach itself. Was more suspicious of the OE yellow top coil, swapped on an NOS Motorcraft coil I had siting on the shelf and sure enough the tach was back to normal.
The interesting observation was, while on the test drive with the Motorcraft coil I noticed RPM's were 200-300 lower than with the "bad" yellow top. Today I swapped on another nice used OE yellow top coil and on the drive it was also reading 200-300 RPM lower exactly like the Motorcraft coil.
My question is...could the slightly higher RPM's from the "bad" yellow top be an early indication of a failing coil? Or just a weird coincidence?
That is an interesting observation. I would like to hear the experts chime in. My RPMs always seem to be a bit higher than what they should be for the gearing. I have a Flamethrower coil in mine.