Shifting problem
 

My 89 Ironman with suntour gpx components is having a slight shifting problem. It shifts from 1-5 great but when I shift into 6th gear after 5 seconds or so
it wants to slip into 7th gear. I don't want to go changing settings by trial and error and screw things up even worse so can someone give me some ideas what to do?
BTW I just put a new RD cable on a couple weeks ago.

I think I figured it out. When I put the bike in 7th gear there is a
small amount of play in the cable. Being a new cable it must have stretched a bit.
It's too fricken hot out for me to go out and adjust and test it. I had just got in
from a half hour ride and I'm soaking wet. The heat index was 108 here.

Where is here? You can put your location in your profile. Mine says New York City but I’m in Ulster County this weekend, and it’s hot as blazes.

Small town of Mt. Blanchard which is outside of Findlay Ohio

Yup, probably stretched cables. I usually tweak the cable tension every week after installing new cables until they stop stretching. Usually takes about three times to get it set and should stay that way for months or years.

Be sure the downtube lever for the rear shift is set appropriately for Suntour and compatible freewheels (including the SunRace 13-25 and 13-28). There's an alternate index shifter setting, just past the friction mode setting, but it's not for standard Suntour Alpha or SunRace freewheels. It'll shift to some gears in that alternate position, but it won't shift across the entire freewheel.

Refer back to that set of instructions I PM'd to you a couplafew weeks ago, for adjusting the rear derailleur. The limit screws are critical -- you don't want the RD to shift past the smallest or largest cogs, which could be disastrous. Then tweak the indexing.

While it can help to make basic adjustments on a work stand, I usually find it's necessary to ride the bike for finishing touches to shifting. Or check the bike on a resistance trainer like a Cycleops -- it mimics road riding pretty well and can help iron out shifting glitches.

On adjustment rides I'll keep a multitool in my jersey pocket or seat bag. Sometimes I'll tote a couple of Allen wrenches with longer shafts, since some multitools are so short it's a hassle to reach some adjustments.