Bicycle Mechanics - Bad Shifting on a Trek 400

Before I dive too far into replacing parts, etc., I will seek advice.

The problem: Trek 400 with downtube shifters labeled "Shimano Light Action," I think 7-speed. Anyway, the lever keeps slipping, and will not stay in gear.

I am used to lever-style shifters (down tube and stem mounted), and understand how they work. The wing nut/tab that I would normally tighten seems to be bottomed out and will not turn further. I have not forced it, as I don't want to cause more problems.

With the Shimano Light Action levers, are there any known problems people have experienced? Is there a big difference in these shifters vs. other makes/models? Does anyone have any good advice?

I'm going to take it apart tonight, and see if something is misaligned or missing. I'm hoping it is nothing major and does not warrant replacing. Thanks everyone for the advice and help, or if this has been answered, I will search again.

You should take the downtube shifter apart and clean the bushings then resemble but do not lubricate the bushings but you can the screw; also some want lubrication on both faces of the lever where it sandwiches between the washers; it sound like your heading that way tonight anyways.

Update:
I took the lever apart, and found a new problem. There is a washer with teeth in the lever, that is supposed to mesh with a similar toothed part in the lever mount on the frame. Several teeth are broken off of the lever side. Is this a critically broken part? It's hard to explain, and I don't have pictures.

I cleaned everything and got it back together, and it is only marginally better. Any further suggestions? Could a small washer be added to help? It seems like the mounting screw is bottoming out before it tightens enough.

Nashbar had some NOS 105 7 speed DT shifters last year for $8 though I haven't seen them lately. Someone should still have them try ebay. If not consider bar-cons or the sora 7 speed sti's are still around but I'm not sure you want to drop $150.

here you are they are A400 105 level 7 speed index or fricition shifting

btw ebay is having a thing for $10 off shipping for poeple who don't ebay often enough so check your ebay messages.


Edit: I erased the link those were 6 speed at icycle.

I've looked into the Sora shifters, and I think they'd be a good economical upgrade. Do they need a to be matched with a specific Shimano derailleur? I would assume there are a couple that would work with the brifters, but I can't remember off hand.

I'll check ebay for some replacement downtube shifters, and mull over my options. Thanks everyone for the help, and I'll keep you updated. If anyone else has ideas or suggestions, i would appreciate them.

Anything to look out for on the Sora shifters? Good points, bad points, who likes them, hates them, etc.

Sora shifters work fine, but you lose any ability to trim the front derailleur. Are your Light Action shifters indexed or friction, if indexed how many speeds? I may be able to help you out.

They are indexed downtube shifters matched to a six-speed freewheel. They have "s424" written on them, if it means anything. I'm not sure if i'm going to upgrade or switch over to a seven-speed freewheel yet.

Thanks for the help Ruppster. Let me know if there's anything I can help you with!