Bicycle Mechanics - rear derailer adjustment is shifting

On my 1990-2 Diamond back mtb the rear derailer fine tuning center adjustment drifts so I need to readjust it every 2-3days. Its Shimano Delore derealier and GX cogs. (I'm not exactly sure about what model termonology applies to, but these words/codes are on the derailers

So its pretty easy to re-adjust (sometimes if I'm lucky I can do it while riding, but can get it more precise with bike on stand)

Anyway - the brakes have a lock nut on them - why doesn't the derailer?

I'd like to put a lock nut or loctite on the adjustment barrel, but why wouldn't it have one like the brakes do? Is there something else that is drifting? I haven't yet gone thru the trouble of marking the adjustment barrels to see if they are moving. I know the cable is not slipping. Do old/worn derailers drift easier due to looser threads?

just posting this in case there is an obvious thing to do first that I am not thinking of.

Al

I'm guessing that your talking about fine tuning the shifting from the bars. When you say that the adjustment drifts, what happens? Does the chain start skipping? Shifting over the larger cog? Not shifting at all or to slowly? It sounds as though you may just need to do some adjustment at the derailleur itself instead of just at the bars. How far out are the barrel adjusters dialed? If it's a lot, you could just tighten them back until you've got about two turns left to go before being fully tightened and pull some more cable through the pinch bolt at the derailleur. This might help if the barrel adjusters seem to spin back to an original position after you move them each time. They shouldn't be turning on their own if your shifters are in good shape. It could just be that they are gummed up or your cables and housings are shot. When was the last time they were changed?

-Moab

You might want to pull out the cable and see if it is fraying at its head. Make sure the pinch bolt is tight. Failing housings can cause this problem as well, thay start to push thru the ends.
If it is just the adjuster turning, wrap a little teflon tape around the threads.

Thanks all

There is an adjuster both at the shifter and at the derailer - I fiddle with both, I put the one on the shifter in a position so I can screw it either way, then adust the derailer one so its centered. Then I can fine tune using the shifter adjuster later. I am only talking about fine tuning - that is to make it as perfectly adjusted as possible to make as little noise as possible in all gears. It never gets so far out that it shifts to the next gear or won't go to all cogs - although I can turn the adjustment enough to do so.

The bike is certainly due for a shifter overhaul, replacing cables, etc.

Al

Usually, at the derailleur end, the barrel has "detents", so that it stays in place at every, say, 1/4 turn. If you need to adjust it between them, I imagine that it would vibrate into one of the detents eventually. If you need to make that fine an adjustment, just put it in the closest detent, and make the fine adjustment on the barrel on the shifter. If that doesn't hold, then yeah, a little teflon tape on those threads will help.

The Rev. has a point, though; your cable may be fraying/stretching rather than the barrel rotating. Easy way to determine which it is: mark the positions of the barrels with a daub of paint or some such (even "white-out" will work), and ride it 'round, 'til the symptoms return. Check the positions of the barrels, relative to where they had been. This'll also tell you which barrel is slipping, if indeed one is.

Well, I just took a break from work and went out and checked it. There is a spring loaded plastic housing around the derailer end adjustment barrel, I pulled it away (along axis of cable) to then turn it and the plastic housing cracked. So I guess that may be part of the problem, I think this is the spring loaded housing that is designed to lock the barrel in place.

So the question I have is can I just replace this part or will I need a whole new rear derailer. I hope the former as I am saving money for a new bike.

Al