Gear is grinding.

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slightly off-topic -

in this image it appears you may have some extra unnecessary friction in the cable path caused by ferrules which are slightly too large to seat in the recesses of the housing stops


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The braze-on on the chain stay may be sized to require a step-down ferrule.
Also, to me it looks like the cable housing between chain stay and derailleur is too short. Compare the image in the "manual" you posted.

Actually I think the original way was not step-down ferrules, just regular single-diameter ferrules but much smaller. Maybe not made anymore (I'm way out of touch with the current offerings) but I have some ancient Simplex housing and ferrules and they're tiny by modern standards.
One reasonable way to git 'er done is to use regular spiral-wound housing with no ferrule, just grind the ends nice and flat and de-burr the holes. Maybe not as finished a look as a proper ferrule, but functionally just fine as long as the diameter matches up so the housing is guided in straight.

Mark B

For sure the old Simplex mech's do not need ferrules.

With the ferrules removed, the housing might seat sufficiently better so as to make good with that actual piece of housing.
But having a correct step-down ferrule at the chainstay stop might really improve the cable path if it holds the housing in line with the cable.

This thread was worth reading just for this one piece of advertisement. Love where it ends: "what is a touring gear?" The term is "meaningless" except that it describes a "gear" used by Cyclists "who want to go places."

It's hard to say it much better than that.

Regarding the grinding noise, I have no experience with this Simplex but it seems to me that chain line would be exceptionally critical with a long pulley arm that is oriented like this, almost pointing at the chainrings. The chain will want to drag the lower pulley sideways until the pulley arm is pointing at the chainring.

My experience with the shorter arm pull-chain Simplex derailleurs is that they are quite finicky about chainline, optimized perhaps for 4 speed freewheels.
Brent

Yeah, that's about a simple and obvious as you can get. Maybe they got school-kids to write these.

There is no cable binding. The ferrules have no bearing on the chain chatter.

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certainly did not intend to suggest that they did

note me message commenced "slightly off-topic"


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Thanks. I had a torturous day with this thing. I listened to what you said about chain-line, and tried putting a spacer behind the freewheel, trying another (just-as-used) freewheel and moving the front chainring outboard a bit. I tried bending the body of the derailleur to get it more precise and I tested every chain link for a frozen one. It's probably something simple like a worn chain or worn sprocket, or both. Thanks for attempting to answer my question, though. It gave me something to think about. It seems to me that the significant question is: Why just the low sprocket? I looked closely at the chain and it seemed that when I got some chattering, it was when the chain looked like it wanted to jump off the side of the lower (guide) pulley. I made an appointment at my local co-op, who are the only ones who know how to work on older bikes, but that is not till May 12. I think I'm going to try my luck with putting on my Benelux Mk7 after I calm down and relax a bit. I can show you a video if you think that would help.

After dealing with this enigma, I am "slightly-off-rocker".

This is only a hunch, not based on any real experience: When the chain is on the largest sprocket the pulley cage is going to be at it's most horizontal angle. It might be worth trying to add as many links to the chain as the difference in tooth count between largest and second-largest sprocket. This will lower the angle of the pulley cage and just maybe solve your problem.
Brent

This sounds like a good idea. I'll try it today. Thanks.