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I have an old french touring bike (from the 70s or 80s) with downtube friction shifters.

I recently had it serviced, and ever since there's been a constant pressure on the rear gears which means they have a propensity to shift into the highest gear. If I shift into a lower gear and let go of the shifter, the chain works it way back up the gears. I can generally maintain being in the second highest gear (which is the one I use most often) but it still requires periodic movement of the shifter as it moves its way forward.

It's possible that before the service the mechanism just wasn't lubricated enough for this to happen.

Does anyone know what's causing it and how I can fix it? I'm afraid I'm pretty much a beginner.

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Rode downtube friction shifters bikes for a while. I can see two possible causes : either the shifter was too lubricated, or the screw isn't tightened enough. Try to tighten it more, but pay attention not to tear the screw. Usually the lever is pretty rigid when moving. If it's easy to move it to the point it moves alone, it's probably because it wasn't tightened properly. If this does not work, you'll have to unmount it and delubricate the parts a bit.

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    I've treated a similar 'disease' on my old bike with a tiny glob of carbon mounting paste where the lever touches the flat base. Washing the grease off with white gas helps, too! – Carel Jan 11 '17 at 11:07
  • @Carel Nice tip ! – Stanislasdrg Jan 11 '17 at 11:50
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    I'll note that it's fairly common for the shifter to have a "thumbscrew" style screw holding the lever in place. This is to allow the shifter friction to be adjusted while riding. – Daniel R Hicks Jan 11 '17 at 13:07
  • Many thanks for the answer. I didn't think it would be so simple, I thought it would be a spring-loaded derailleur or something. @DanielRHicks was correct about there being a simple thumbscrew on the lever that needed tightening. – thosphor Jan 11 '17 at 14:15
  • @user3087409 - Bikes are simple until the manufacturers try to make them complicated. – Daniel R Hicks Jan 11 '17 at 20:05
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You already have the correct answer for you (tighten up the friction screw) but there's something else you should look at: balanced stem shifters.

Regular stem shifters hold their place by friction -- that's why they are also called friction shifters. There's a fine line between not enough friction to hold the gear, and too much friction which makes shifting difficult. Especially if your derailleur springs are strong.

Balanced shifters have a spring in them which counteracts the derailleur spring. So you don't need as much friction and shifting is as light as a feather.

I got my pair from my local bike shop owner, who liked me. They're vintage Campagnolo but Shimano also made them. Ask around, your local bike shop might have a set in their vintage / old equipment cabinet. My set were put on my 1975 Peugeot UO-18.

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