Gave it a tune-up and now it's running rough.
 

It's very sluggish to accelerate and backfires now. Feels like the timing is off.

I replaced the plugs, fuel filter, cap & rotor, air filter, wires, and injector o rings.

I had trouble getting the replacement cap to lock into the grove. Had to remove the round black shield to make it fit.

Is it possible I might have inadvertently fixed a vacum leak that might have changed the vacum advance?

It's a 73 1.7 D-Jet.

1-4-3-2
as in the firing order of the plugs. I bet you got two wires crossed. only way it will back fire.

I would replace that cap with one that fits right. If it is not seated all of the way, it can cause problems. Hook up a vac gauge, and see what your vac is, then check your timimg. If you were fighting the cap, you could have the dist. Also, check firing order.

I checked the wires today and they look good. Some times, it runs fine..... other times, not so much. I'm going to check the timing today.

If it varies like that, maybe a vacuum thing. When I got my car, it sure ran a lot better with new vac hoses.

You didn't mention what prompted the tune-up, but how long since the last valve adjustment?

Since I don't like using the "replace with new part and see how it runs then" method of diagnostics, and diagnosing DJET system can be challenging, I found that all my "tune-ups" would start with a valve adjustment. I drive my car moderately hard, so it was good to keep an eye on what was happening under the engine tin as much as above it.

Just some added things to think about - and muddy the water ; )

I would suggest looking inside the dist. cap closely and check to make sure it did not get damaged. I would say to make sure the correct rotor was used and/or that it was seated correctly as there are more than one rotor depending on what distributor the parts are for. One rotor is slightly taller than the other and the same with the dist. cap as it should have fit exactly like the previous one. Just doing what you mentioned will not change the timing unless you replaced the points but you did not mention that.

Did you hook up the hoses exactly as before as I can not remember it the 1.7 D-Jets just used a vacuum retard like the 2L engines do so hooking up vacuum advance will most definetly affect the way it runs.