My boss has a early 70's Raleigh Super Course that doesn't stay in gear and he asked me to look at it and get it running correctly as cheaply as possible. It has the good ol' plastic Simplex RD and the associated shifters and what not. In riding a short test ride it would generally operate correctly in gears 1-3 & 5, but in 4 there was a repetitive chain skip that got worse under heavier load. I could not see any damage or significant where to the actual sprockets.
My questions are:
First, does this sound like a symptom of that RD?
Second, the only new drop in replacements that I have found are either the Simplex SX101 or the Shimano Tourney as they have built in hangers. Anyone have opinions on what to go with or know of a cheap adapter claw/RD combo that would work out better?
Skipping in particular cogs under load is usually an indication of a worn cog. Replacing the freewheel and the chain would be the first things I'd try. If the RD is throwing the chain around okay, it and the shift lever should be okay.
I find that skipping problems are rarely related to the derailleur itself. It is usually the shift levers, chain or freewheel.
The fact that it occurs on the 4th gear makes me think that the problems is related to the cog itself. It could be a damaged tooth (bent, chipped or burred). However, it sounds like the bicycle has probably been ridden in the 4th cog for most of it's life and that cog is excessively worn. When a new chain is installed it meshes properly with the relatively unused gears but poorly with the highly used cog.
It could also be that the shift lever tension screw has backed-off but this generally affects all gears. However, this is the easiest "fix", so check the torque anyway. It may take a bit of fine tuning to have the screw tight enough to maintain lever position but not be too hard to move the lever.
It could be a stiff link in the chain, but this generally is worst on the smallest cog, as requires the links to bend the most.
I suggest you spin the crank slowly to see where the skip is happening. Mark the cog and chain with a dab of paint, marker or tape. See it repeats in the same spot, either on the cog or chain. If it does you have a specific problem with a tooth or link respectively. If not, it's a general wear problem and most likely the freewheel, especially if the chain has been replaced.