1973 450sl Misfire Issue
 

I have a 1973 450sl that ran perfectly until I decided to do some preventative maintenance. *facepalm
Thursday night, I was looking it over and noticed that my ignition rotoscoped was a bit worn. Nothing bad, just maybe a few years old. I installed the new Bosch one that I had ordered. With the new rotor, it ran fine under acceleration, but at traffic lights, it would stumble a bit and sometimes stall, always starting right back up.
Friday morning, I figured I'd go ahead and put the rest of the new parts on. Plug wires (Bosch), distributor cap (Beru), and points. Since this, I have the misfire from hell, and backfiring while driving. The original cap had part of a write that broke off in it while I was removing, so I cannot just swap back to what I had on there.
My question--I suspect the cap to be the culprit. The wires were put on identically to how they were on the old cap. I'm sure the order and locations are correct. Have other people had issues with Beru caps? The fit doesn't feel as tight as the Bosch cap that came off of it. I can move it around a bit with it clipped in place.

What are your thoughts?

I think it's ok if it wiggles a tiny bit if memory serves. The more interesting question would be if you used the right rotor and wires.
You see, the 4.5 has a weak (by modern standards) spark system, that requires non-resistor wires, and a non-resistor rotor. There are resistor rotors and resistor wires available. The OEM set had resistor plug ends, but the wires were solid copper.

Take a look at the ohm measurement on your new wires and compare to the old. Make sure your rotor is also not a resistor rotor.

Lastly, make sure your firing order is right. This isn't an American V8, the firing order is 1-5-4-8-6-3-7-2, and 1 thru 4 is all on the passenger side, and 5 thru 8 is on the driver's side.

The rotor is the complex style with the spring mechanism, same as what was on it.

also look over what you did, did you set the point gap correctly? Double Check the wires and make sure they are pushed all the way in on the cap, especially check the coil wire .

Any chance you moved the distributor in the process of replacing and now timing is off ? Did you check it ?

A quick clarification of Tomguy's post. Sitting in the driver's seat, facing forward, cylinders numbered one through four are on the right hand side with cylinder number one at the front of the engine.

A bit pedantic I know but this covers the car being either LHD or RHD.

The misfire, backfire, and a general lack of power would all align with weak spark and/or timing being off. This is why I suggested checking those items. Replace the rotor back with the original, and see if that makes an improvement. It will also allow you to ensure the carbon bush on the top of the cap did not break during cap installation.