I've fixed/flipped/restored a lot of bikes, mostly vintage, but sometimes I run across something that I feel like I should know but don't.
I done many 20-25 French restorations, but the Simplex stuff is usually toast and I replace it. And eventually the French bikes started shipping with Japanese parts. This derailleur looked good, but I can't get it to behave. The top pulley is too close to the freewheel. It handles the low cog but it can't move up/inward without crashing into cogs. If I hold down the pulleys, second photo, I can shift up to the large cog. I don't see a B-screw on this design or any other way to push the pulley cage down. And btw, this Simplex RD came on this bike! But I disassembled it a decade ago and hung the frame on my wall for a future build.

I think I've read others having that problem but never seen it myself until today. I had plenty of Suntour RD's but mistakenly donated them to MikesBikes Africa drive, and they are long gone. I've got two choices: this Simplex and a too-modern looking Acera, hardly the right thing for a Peugeot Mixte. Still, if I have to...
3rd choice, there's a swap meet Sunday morning, if I'm still stuck.

Idea's?

Ask in Classic & vintage where the guys work on this all the time

I would assume that your chain is too long, since the RD cage is hitting the chainstay. You have both limit screws, so nothing missing there. Put your FD in the small ring, and your RD on the small cog, and shorten the chain until there is enough separation.

Why is the cage outside the dropout? Something badly bent or just not adjusted?

If the chain were too long the der would be pivoting rearwards, pulling the cage and pulleys away from the cogs not into them. Most likely what's going on is the B pivot spring has either broken or one of its ends has disengaged from its insertion hole. I've seen this on a number of Simplex ders over the years (although not for quite a while) and never found a repair method that stayed. I learned to not take apart the B pivot, or even try to adjust the spring tension as doing so has caused this for me a couple of times. Back then a replacement der was really low cost so wasn't a big deal. If you do move to an Asian made der double check that the plastic shift levers will pull enough cable to have the der shift through its entire range. Some of those older Euro shift levers had small cable barrel diameters and many of the first wave Shimano and SunTour ders wanted a lot of cable pull... Andy.

Has the upper pivot spring become disengaged? If there is no spring tension on the upper pivot, the pulleys will be pulled too far forward; the spring has either come out of hole in the stop plate or tension bolt, or is broken. The spring (part #2973) in the picture below, has a pins that fit into the plate (part #2978) and tension bolt (part #2977) and must be preloaded to provide tension.

I used the Big-Big method, but only add 2 links.
"Place the chain over the largest sprocket and the largest chain ring and bring the ends together without routing the chain through the derailleur. Count four more chain links to the length and detach the links you don’t need at that point."

not sure what you mean by cage. I've always seen and always put the hanger outside the dropout, except in an odd case where I spread the dropouts to 130mm for a modern wheelset and had a RD reach problem.

thx for the warning on cable pull. Most of builds become Frankenbikes so I often am scratching my head when parts don't play well together. In this case I already abandoned two FD's, one due to reach (I'm not using the stock cottered crank), the other due to lack of cable stop. The 3rd one is working. I currently have the stock RD and stock stem shifters, but have a large supply of vintage shifters to try if I run into this problem.

thx for this great detail. Now I won't go in blind if/when I take this apart. I always have problems visualizing (and remembering!) if I take something intricate apart.

If it helps, I would check to be sure part 2978 is still there and in the right place. The little tab on the right side is what stops the derailleur from swinging too far.

RATS! Man those Simplex derailleurs are great! Till they are not.

On those old stamped thin dropouts I weld the bottom half of a Suntour Claw. Those extra few millimeters gave me enough clearance to run bigger freewheel cogs.

Derailuer Hangar Welded - "Puch"

As for your problem I think moving the derailleur forward as much as you can will give you more clearance on the cogs. Certainly the springs in the derailleur are not as strong as they used to be 40-50 years ago. This is sad. I do remember back in the day Simplex derailleurs were fast and furious. But with plastic parts, and then the Campy Nuovo Record, Ha, its all history BOSS. I am sure you know this...

that is a badly designed derailleur, made for corn cob freewheels, and it will not work on that large of a low gear.
they didn't work very well on corn cobs.
the only saving grace for them was non-index shifters allowing fine adjustment on the fly

slap the acera on the bike until you find a better choice.
there is no shame in upgrading away from a simplex.
a shimano C10 or LX/DX would be a good look on that age bike.
there are some old school Shimano Tourneys that would work too.

he was tricked by the camera angle.

cage refers to the parts that "cage" (hold) the pulleys

No, they were designed for a freewheel with 28 teeth (at least) on the big cog. All of my Simplex rear derailleurs work fine with (say) a 28-14 range. Not sure how many teeth the thread starter's freewheel ha, though.

OK, that's what I thought cage meant, but the sentence didn't (to me) because I didn't notice it was on the wrong side, so I assumed you were taking about the hanger. But it now looks glaringly wrong.

yeah, there's the visual of the Acera, but also the inertia of making the swap. Didn't want to remove the wheel and break the chain (again). But that may be my next step. And after all, I won't be staring at the RD while I'm enjoying the ride.

Odd comments:

The Simples Crit/Prestige series of rear ders were rather advanced for their era, by the mid 1970s that era was on the way out... They had sprung B pivots and pulley cages with the cage pivot independent of the guide pulley. They were relatively light weight and not too expensive (the main gripe I had was their stiction in the parallelogram pivots when new). Significant improvements compared to the Campy and Huret offerings back then. The book, The Dancing Chain, should be on everyone's must read if they want to talk about der designs.

No reason to break a chain to uninstall a Simplex rear der and install an Acera one, both have bolted pulley wheels in their cages. But the longer cage of the Acera MtB design will likely want a longer chain... Andy.

the der. can be swapped in without breaking the chain... shift bike to small/small gears, remove back half of cage from old der., derail chain off of front rings, mount new der., remove lower pulley only, route the chain through pulley cage, install pulley.
it gets easier with practice... and enlisting help helps too.

a nice fresh chain with a Quick Link makes it a much easier process.... hint hint

Installing Rear Simplex Derailleur on 1980 Peugeot Super Sport

The Prestige derailleurs actually work very well when new, and handle up to a 28T sprocket. They performed better than most other derailleurs on the market at that time, and were considerably lighter as well. The problem is that they don't tolerate neglect. Grit eventually erodes the pivots, so they become sloppy and shifting performance suffers. And the Delrin plastic become brittle and failure prone after prolonged exposure to UV light.

The cage is the part(s) that holds the pulleys aligned.

i recall riding a Peugeot back to back with my Suntour equipped Mitzutani Super-Lite factory race bike... Shifting was kinda vague, and suffered from hesitation, then needed a tweak a minute or so later.... Wes' PX bike's chain always seemed to flop around over bumps too.
i concluded that simplex sucks back then.... and you acknowledge that they don't improve with age, unlike many french wines.

Suntour also made some not-good ders in the day... i have one that i have tried to mate up to a few projects... it keeps landing back in the suntours bin, in a bag.. now with a couple extra notations on it.

my only campy bike of that 1960s-70s era was also my first road bike, and i had no chance to compare it before it got stolen.. a Lygie Dura-lumin bike... dressed in my favorite blue...the r. der. was a gran turismo with that crazy loop-de-loop cage... it would shift, eventually.

cassette design also plays into shifting... a straight cut freewheel will always be worse than a more modern cogset.
i don't recall what freewheel was on that bike.. and i should add that i rescued that bike from a chicken coop, complete with a pound of free fertilizer on it!

i have an immaculate looking simp-der in my bins... it is sloppy feeling, but the plastic hasn't cracked... yet.

that odd cage pivot placement may be why Wes' chain always flopped around, IMO. i know he never dismantled it, since just changing a sew-up baffled him.

again, thx for the pull warning. When the Simplex stem shifters did not cover the entire 6-spd freewheel, I did not hesitate to replace it with a Shimano I had. That was coupled to a Suntour RD I picked up in yesterday's swap meet (and I think I got the only vintage one at the entire meet!), and is now working. Everything came together fairly quickly with these parts. This is a 56cm and fits and feels great. Mounted some modern wheels (126 O.L.D.) with wide rims and 27x1 3/8 tires. Love it!

Nice build! I;m not surprised you like that bike -- Peugeot knew how to make their low end bikes ride well.