My 79 (all stock) started running rough as well as started having erratic tach behavior.
I replaced:
condensors
plugs
points
rotor
cap
voltage regulator
coils
wires
fuel filter
oil filter
oil
treated with seafoam

I tried to replace the alternator as well, but was sent the wrong part. The tach still wont work at idle, switching instead to voltage mode (I assume). Once the car is rolling it registers normally.
The idle is rough, with pops and rumbling. It holds ~800rpm fine, it won't die, but it is rough.
Driving the car is fine under 3000rpm. The motor wont pass 4000, and backfire at every shift, no matter how soft.
The #2 trailing plug was black and fouled, the rest werent black but didnt look healthy either...
I haven' re-checked the filter since seafoam, it might have been clogged with debris.

But why wont it idle smoothly? Would a bad alternator cause this? Should I start looking deeper into the carb? I check the accelerator pump, it squirts fuel just fine.
Im not sure how to check the secondaries, but they wont affect idle....
Any thoughts? Despite all this, the car has been easy to work on. Im liking it quite a bit.

 

Possibly running too rich.

 

any other thoughts? i doesnt smoke at idle or driving.
i still need to check the secondaries to see if they are stuck, but im not sure that is the only cause...

 

Check the trailing igniter.

 

79 w/ points.
no igniters, if i remember right

 

Just a thought.
Have you tried disconnecting the Tachometer ?

 

might fix the tach, but the motor still isnt running right....

 

My first suggestion would be to ditch the points ignition and switch to electric ignition with a FB dizzy and coils.
The tach signal comes from a ring connector on one of the coils. Take the rubber caps off of the poles on the coils and make sure that the retaining nut is on there tight and you are getting a good connection to all of the wires coming off of the poles.
The idle problem might also be a small vaccuum leak around the carb or just a bad tune on the carb. You could try adjusting the idle mixture screw (it is the lower, smaller one in the middle of the carb on the drivers side.
As far as the backfiring goes, mine did the same thing when i had the stock exhaust. It took me a city block to get the car to 6k. The exhaust is no less than plugged, and if you can, switch to a header and ditch the thermal reactor setup and the car will roar.

 

The tacho signal comes from the negitive side of the ignition system.
If the tacho fails, it can ground the coils to earth, grounded coils won't
produce spark.
If you disconnect it the tacho won't work at all, but if the engine then
runs OK you have a problem in the tacho. If it dosen't at least youv'e
eliminated one more thing.

 

I dont see where you set the dwell for the points... If the dwell is far off, the tach doesnt work...It also means your not getting proper spark.

If I recall 58 degrees for L and T

eventually you will want to change to electronic ign of some form as the points burn up fast on this car and dwell goes off mark just as fast and you need to constantly reset/change the points