Positron II derailleur adjustment
 

I have s Bridgestone kabuki SYD. I replaced the rear derailleur (suntour V-GT Luxe) with a Positron II. When shifting to largest rear gear it throws the chain.

I looked online and I can't find anything on adjusting it. I never had a rear derailleur with only one screw.

Any information would be great.
Thank you

Don't these things usually come with giant pie plates?

What do you mean?

Dorks discs, or spoke protectors. The big disc that goes behind the freewheel to prevent you from throwing the chain into the spokes.

Duh, lol

Thanks

What was wrong with the V-GT-Luxe that led you to replace it with a Positron? I'd consider that to be a downgrade, actually.

Since this is a conversion are you using the proper uniglide stuff or at least modern Shimano stuff?

The Positron II does not have limit screws, so it's critical that it be used on the freewheel for which it is designed. One that has slightly less spacing between cogs would cause the problem you describe. It's also important to use a Uniglide chain. I don't know why you would go to a PII, which is not only obsolete but is so for good reason.

I was riding a huffy breakaway that hat the positron on it. I got the kabuki off Craigslist. The v-GT was missing two screws and decided to put positron on until I decided what upgrade to get.

The chain is original and I will be replacing that .

Using both an incompatible freewheel and chain are the likely source of your problem. You adjust the derailleur by loosening the screw, moving the derailleur to the center cog, and then tightening the screw down on the shift wire. The derailleur then moves the proper amount at each stop for the freewheel spacing that it is designed for.

I did that.
The only problem is when i shift to largest rear gear it jumps between spokes and gears.

I ordered tools to fix it

What modern inexpensive derailleur would y'all recommend?

I guess I was wrong when I thought derailleurs where universal

I mentioned how to adjust only as a point of reference. Adjusting does no good if the chain and especially the freewheel are not compatible. Likely the spacing between cogs is slightly smaller than the derailleur is designed for, and by the time you shift to the large cog the difference is enough to send the chain past the cog into the spokes.
As for replacement, almost any low end derailleur will work if paired with a normal lever and cable.

Bummer that your V-GT Luxe was missing parts. Those are great derailleurs. If you're using friction shifting (and you are if the original is a V-GT Luxe), most derailleurs will work so long as the cage is long enough for your gearing. There are some exceptions, though, and you found one.

As for inexpensive modern derailleurs that'll work... If you don't mind a cosmetic mismatch, the Shimano "A" series derailluers (Acera/Altus/Alivio) are dependable units that work well with older drivetrains. The Acera M360 is available brand new under 20 bucks.

If your V-GT Luxe is in decent shape other than the missing screws, I'd be happy to made a trade, sending you one or two working, compatible derailleurs in return. (I'd have to check my parts bin to see what I have available.)

Positron, and all Shimano indexing for that matter, required system integration. The shifters, the chain design, and the cogs all played a part in proper indexing. That's something that's still true, it's just that everyone now makes those parts for indexing.

I kind of like the junky chrome Sunrace that's a copy of the dual sprung Shimano Skylark/Tourney. Dirt cheap, works ok, and looks good on a vintage bike.

cny-bikeman, thank you, I misunderstood.

skyDog 75, message sent .

Positron 5/6 speed fix.
 

Old thread but maybe found answer to original question. Next to no info on internet about these deraileurs. My six speed Varsity had opposite problem. It would not shift into sixth gear. I found a small red plastic piece under ratchet that could be set to five or six speed. Mine was set to five speed. As soon i tried to turn with screw driver it fell apart. But then I had six speeds! If you have a five speed with chain jumping, count the notches on ratchet. I could be trying to jump into a gear that is not there.