have an old ten speed from 70's i've been trying to tune up. I can't seem to shift into the middle cog from a smaller cog (downshift?). I can shift from a lower gear (bigger cog) to it no problem, but if I'm in a one of the first two i have to shift past it and then come back down. friction shifting, older sun-tour derailleur. Any advice?

Thanks!

How old are your cables?
Could the housing be worn out?
I would try new housing and cables, as well as clean and oil the derailleur.

cables are brand new. most of the housing is too. i havent noticed the derailleur jumping from one position to the next when shifting the cables. it just doesnt seem to catch the 3rd cog and you keep pulling til it catches the fourth. i will have a closer look though, but I'm pretty sure it moves continuosly

check the teeth on the cog ,one or more might be bent . being friction shifting it should be working ,unless you not feeling it.

thanks for the tip, no bent teeth however. I made this video of my issue as it happens hoping maybe somebody will see what is going wrong and how to correct it.

View My Video

It hard to tell from your video but maybe your derailer 's hanger may be off a little , if it is take it to your LBS and have them look at it and adjusted it . Other that your cable might be to tight or there something wrong with the housing and will need to be replace . it only take one piece of housing to mess thing up .

thanks. i inspected the shifting carefully to see if the dr was jumpy, but that was not the case.
I ended up adjusting the b-tension screw. that helped a little but it was still very hard to hit the gear. there is a big jump between 2nd and 3rd cog teethcount wise. i decided to switch out the freewheel and now it shifts good (problem one was ancient). my new problem is blown tubes. cant seem to get the rear tire to stand much more than 50psi pressure before it pops off. tried two different types of tires.

Post a pic of the bare rim.

Video was a trip - reversed. Looks like a left-side drive train. Did you shoot it sitting in front of a mirror? Looked like the teeth were not catching on the middle rear cog. Backing out the B-screw probably improved but not fully. Other other thing I might have tried would have been to slide the wheel back further in the dropouts. Hard to tell if that will help. This might give the top RD pulley more wrap on the chain and provide more lead distance to allow the chain to shift. but the other thing to check is bevel on the FW teeth. Sometimes the cog is flipped around and the bevel won't catch the chain.

ok. what angles do you want to see?

wow, I just caught that reversal deal. it is the bizaro version of my den! I didnt use a mirror, shot it with the built in cam on a macbook air. that is wild. I will check the video to see if it's like that or tinypic flipped it.

your second point about bevel, i'm not sure I follow. how do I check this?

Oops, never mind - that post was meant for a thread about a tyre blowing off : /

since the bike is from the 70's ,there a good chance the rim is steel and they don't hold up well to high pressure ,like 70 psi or higher . does the rim have little bumps all the way around it on the outside and both sides ? the bumps are there to help with braking ,but they don't do much . there no lips on the inside of the rim right ? which today rims have to help to hold the tire in place . the only way to work with this is to keep the pressure low like to 50 or 60 psi or replace the rim with a newer one with the lips .

bikeman, thanks. that is pretty much what I have gathered about the rim. it's alloy, but no inner ridge/lip. apparently some have luck with them at higher psi and certain tires. but I have not

You mean that wasn't intentional? Indeed, what a trip!

Some older freewheels had sloping teeth. The edges of the teeth were beveled so the teeth were lower on the outside facing away from the spokes and higher on the inside. Depending on the make, it may only be on select teeth on the cogs, such as two shorter teeth on the cogs. Sometimes 3 shorter teeth on bigger cogs. But the bevel would allow the chain more ease to slip over the cogs and then drop and engage. If the bevel is flipped around, the chain might have to go just a bit higher. But if it's too tall, the chain bends, can catch and goes a bit too far and catches the cog 2 shifts over, instead of the next one.

apparently the webcam is set up to record a mirror image, odd