Ok here we go. I have an 85 coupe with a 4cyl. For a while now I've been trying to get the Tach working again( needle resets @ 0 then pegs when engine starts).

I know the filter is good because I swapped filters between the 4cyl. & my 86SE 6cyl . It works fine in the 6cyl.& the filter from it had no effect on my problem in the 4cyl.

I have aquired 2 used tach assemblies (Thanks Kevin-SunCoast Fieros) with the thought that my circuit board was bad.
The 1st assembly is for a 4cyl.(solid orange line starts @5K RPMs), does the same thing as my orig.(re: pegs).
The 2nd, I think from a 6cyl (solid orange line starts @6K RPMs), works but shows the car idling @ 2-2.5K

So I guess my questions are these...
Are 6Cyl tachs interchangable with 4 cylinders? Is recalibrating required?
If the ignition module was not functioning fully would that cause the pegging needle?
Could it simply be my timing is not adjusted properly? (car does back fire occasionally&has no CAT )

you've ruled out a circuit board problem.
you've ruled out the filter.
you are getting a signal from the distributor cause it makes the needle move, so the wires are probably good.
that leaves the module.
that's the only logic i see.

and yes you can use a 6 cylinder tach on a 4 cylinder car after you modify it.
you can change the resistors (using a pot would be the best way cause you can get it perfectly adjusted)
or you can change the cap.