Shimano FFS and Positron...
 

Is there any reason I can't substitute a decent derailleur for the Positron on my '83 World Tourist? I've fiddled myself blue in the face with that POS and while I've got it downshifting nicely, it just doesn't upshift worth a darn.

Hate to be the bearer of bad news, but no you cannot. This is a total Shimano system approach and no other derailleur to my knowledge will work. The shifter, rear derailleur, r der. cable, and freewheel were set up to all work together. Assuming that this is a 5 speed set-up, the secret is to shift it into the middle (3rd) rear freewheel sprocket. There is only one adjustment screw inside the body of the rear derailleur. You want to center the rear derailleur vertically directly above the 3rd cog. If I remember from about 100 years ago, turning the screw clockwise moves the rear derailleur up toward the largest rear cog, and counter-clockwise down toward the smallest cog. Once you have the rear derailleur correctly centered over the 3rd cog, it should work quite flawlessly . Good luck.

You should be fine as long as you change out the hard wire cable to a regular dérailleur cable. I can't remember if the Positron lever accepts a normal cable. If not, you may have to throw on a different set of stem shifters.

I've got the rear derauler, shifters, 3 pc cranks, bb cups bearings all in very good shape if you want em. I cant guarantee its 100% complete, but darn close. Just cost ya the price of shipping. I stripped all the FFS sttuff off my early 80's varsity. I was just going to toss it...........Then again, it probably wont fix your shifting problem...........

There's a new user on here looking for FFS spare parts... don't throw that stuff out, either of you, without looking for his post!

My Ross Eurotour w/FFS uses a Shimano Eagle II rd with regular braided cable. I don't see why yours couldn't too. BTW... I had to bend the rd upper pulley cage a bit to get it parallel with the freewheel gears to get it to shift both up and down equally. Maybe that's where your problem lies too.