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I have 1979, about 55,000 miles on it. Almost stock (now). I've owned it for 2 years (live in Wisconsin, USA). No winter driving.

This issue started suddenly. I drove the car occasionally for the first year. This summer I removed the stock exhaust ('rats nest') and replaced it with a Racing Beats performance exhaust (header, pre-silencer, muffler). This involved removing Air Pump, blocking off no longer needed ports and vacuum lines. It ran great! I also removed the stock air intake and replaced it with the Racing Beats air intake. More vacuum lines where no longer needed and blocked. It did/does BACKFIRE on deceleration, which was something that didn't happen with the old stock exhaust.

I drive it pretty hard on the road, high revs, etc. Not so hard that I have the police chasing me (yet!). Then SUDDENLY one day I started it with no issues, drove down the road about 1 mile and the TACHOMETER JUMPED ERRATICALLY and went to 0, and then the car RAN VERY ROUGH, with little power. I nursed it back home.

The next day I started it. Started fine. I took it for a test drive, and the same thing happened, ONLY this time when I got it back home it was running smooth. I gambled and took it for another test drive. It ran great at any speed, good power, zero issues. I thought I may have had too much fuel in the carb from starting it and it was just choking itself on all of it.

2 days later, went for another drive. No starting issues, but shortly after driving it, the tach went to 0 and it ran very rough. Same thing, drove it home and parked it. It started fine days later, except the tach isn't showing revs, and it idles very rough.

I've read countless threads on the web, watched YouTube videos etc. I'm looking for help!

-Ignitors...I thought I would replace them, but saw that you can't just replace them with any ignitor (regardless of horizontal or vertical orientation). Which ignitors are the right ones?
-I looked at my in-line fuel filter which I replaced at the same time I put the new exhaust on in early summer (2020). Filter looks good. No crud, no particles. I was concerned about ethanol fuel fouling my lines but that doesn't show up. Carb looks clean too.
-I re-torqed the exhaust to installation specs, at the manifold, and all bolts within the exhaust system.

I'm desperately looking for help! I'm pretty mechanically inclined. I work on all 3 of my cars, and get pretty involved, but I am NOT a mechanic. This is my first rotary too. Appreciate the help!!

Problem sounds electrical & possibly Ignition , suggest you start with the basics ....coils/HT leads etc
Also double check all your Earth/Ground connections intermittent faults are the worst

Appreciate the insight! I just order new ignition coils thru Mazdatrix. I'll start with those and look at the grounds too. Thank you!

Tachometer gets signal from the trailing coil. Yellow wire / green stripe I think. This covers all models '79 thru '85.

If you haven't already done so, check all of your plug wires. Four between distributor and spark plugs, two between ignition coils and distributor.

Your IGNITION COILS are the one's mounted horizontally. Check the operation of both.

Plug wires and ignition coils should be your first investigation!

If you are running the original '79 ignition setup, you will have a pair of contact breaker points inside the distributor - one pair for leading and the other for trailing. Check that these are set up and gapped correctly.

If you have the '80 and later electronic ignition, you still have the two ignition coils and additionally, two IGNITORS mounted on the outside of the distributor.

Thanks KiwiDave! I've ordered new ignition coils, relatively inexpensive, and the ones in there look to be original. I'll look at the plug wires again. I've visually inspected them and ensured they are seated properly already (since I replaced the spark plugs last week). I found my timing marks on the pulley flywheel, and aligned the leading mark with the indicator, expecting to take out the distributor. That probably is my biggest concern, ensuring I know exactly what I'm looking at, and looking for, and correcting anything not right. I do have a Haynes manual, but those pics are ancient and dark.

I appreciate the insight! This is a huge help making sense of everything I've read in the manual and on-line.

You can interchange the the leading and trailing coil for test drive.I would say the leading can be the problem coil.

Do you have a timing light? This is the easiest way to set the ignition timing. You don't need to remove the distributor!

Haynes manual has a very clear diagram and instructions on how to adjust the timing.

Connect your timing light to leading plug wire on FRONT rotor. Loosen the adjusting screw and rotate the distributor body to line up the marks if needed.

Connect your timing light to the trailing plug wire on FRONT rotor. Loosen the two adjusting screws on the vacuum unit. Move the unit toward or away from the distributor body to line up the marks if needed.

Plug wires and ignition coils first and then points and condensors before you look at the timing. If you're not sure of what you're doing with the timing then get someone to show you before you touch anything.

Also, make ABSOLUTELY sure that your plug wires are connected correctly. Everything is marked on the distributor cap and the rotor housing

L1 and T1 go to the front rotor. L2 and T2 go to the rear rotor.

Thanks again! I don't have a timing light, but can get one. I'll take care of the ignition coils, and check the plug wires again. Appreciate the thoughts again!
I'll take a look at the Haynes manual. I'm confident I can do it if it needs to be done, but I'll take care of the first two items before looking at the distributor, and keep the timing last (if needed). I can see from the small bolt on the distributor that it hasn't moved on it's own, meaning it's position hasn't moved. I'm less inclined to think it's timing, more inclined to think ignition coils, and or points, and or condensers.

I'd say swap the ignitors around too.

I've looked at the distributor, I think Igniters were from 1980 on. I'm waiting for new Ignition Coils, new Points, new Condensers, new Distributor Rotor. Hopefully I'll have them all by the weekend!

Appreciate the ideas and all the help!!