My inline front mech barrel adjuster unscrews itself while riding. It's on the down tube so I can tweak it on the move, but when I realise it's moved isn't always an ideal time. For example I shift from the big ring to the middle ring, nothing happens, but I know that clicking the lever again will put me in the granny ring. In gloves I can only adjust it in the granny ring anyway as that's when the tension is least.
It has a spring but that doesn't seem to help; in fact it may make things worse. I've tried loctite (a not-too-hard variety I got as a sample from a loctite rep years ago, but effective on other things). It doesn't seem to help, but that may partly be because the braze-on is likely to have grease in it despite my best efforts. Ideally I'd fit a locking nut but there's very little space, it would have to be knurled, and the thread doesn't look like a normal one.
Yes, it's filthy, but that's mostly from yesterday's 200km ride.
So how can I lock off this adjuster (until I need it of course)?