Bicycle Mechanics - shimano shifting

i have a shimano STX special edition from 1974 brand new out of bag. it is on the bike now and will shift from small to large cogs but once it gets to the big 32 it will not shift down to any other gear the small tooth is 13. the shifters are suntour friction and the front crank is 52/42. I dont know what the chain length should be? help.

Is the max cog capacity of the derailleur being exceeded? If the upper pulley is being jammed up against the largest cog it won't release...?

I am assuming that you have a Shimano STX long cage rear derailleur, paired with SunTour friction shifters. When the chain is on the largest rear sprocket and the largest front sprocket, the rear derailleur generally should be at a 45 degree angle. It sounds like you need to put 4 links in your chain. If the chain is too short, the upper pulley wheel, as detailed in the above post, will jam up against the largest rear sprocket and not allow the rear derailleur to drop down . Otherwise, there is also a B adjuster screw on the rear derailleur that moves the derailleur closer or further from the sprocket that it is shifted into. You could also try working with that, moving the derailleur either closer or further from the sprocket. I would try increasing the chain length first.

ill work the b screw more have already but give it another go. chain length is good idea thanks

it is a long cage

I am assuming that you have a Shimano STX long cage rear derailleur, paired with SunTour friction shifters. When the chain is on the largest rear sprocket and the largest front sprocket, the rear derailleur generally should be at a 45 degree angle. It sounds like you need to put 4 links in your chain. If the chain is too short, the upper pulley wheel, as detailed in the above post, will jam up against the largest rear sprocket and not allow the rear derailleur to drop down . Otherwise, there is also a B adjuster screw on the rear derailleur that moves the derailleur closer or further from the sprocket that it is shifted into. You could also try working with that, moving the derailleur either closer or further from the sprocket. I would try increasing the chain length first.

Instead of randomly adding links to the chain. How about we just verify that it is

a) the proper length
or
b) too short

I dont know what the chain length should be

Would be something to learn wouldn't it? , resources abound on that ..
Buy a new chain , then remove the extra link, as they don.t like trial and error
multiple uses of the chain tool any more .. part of adding all those speeds ..

The STX derailers, all of them, were specifically for compact rings/cogs, meaning 28 or perhaps 30t max in back.

You will need to assure that sufficient B-tension is achieved to run a 32t cog in back, which may necessitate a longer screw or even adding a dome nut to the tip of the screw to keep the lengthened screw from slipping off of the frame dropout's hanger tab. A stiffer, 5mm diameter cable housing can actually help boost the b-tension effect as well.

Chain length needs to be from 2-4 links (1 to 2 inches) longer than full-tight (when using the biggest sprockets front and rear), and likely more if you have rear suspension (which must be compressed to the chain's tightest position when checking chain length).

There was an STX group in 1974?