knobster

Bought a used bike with Dura Ace STI shifters. The downshift on both shifters misses fairly frequently. The rear is worse than the front, but they both do it. I can flick the shifter and it'll then downshift. Is this a damaged component or is there some sort of trick to get it to work properly? I've had many bikes with STI shifters and this is the first one that's done this.

Ingleside

Assuming that the cables are smooth and the issue lies within the shifter itself there are only a few things you can do.
The grease inside the shifter guts may have solidified and now is inhibiting smooth shifting. This is typically the issue with Shimano shifters.
Shimano shifters are not intended to be serviced by the user so all you can really do is try to spray a lite oil like tri-flow into the guts of the shifter and try to free up the engagement plaws inside. Work the shifter up and down the range of gears a few times and repeat it. With some luck it may free up.

San Rensho 

When you say it misses, do you mean it clicks and doesn't shift or it hesitates? If that is the case, put the bike in the stand and increase the tension on the rear der cable until the the upshifts just start to hesitate. Then lessen the tension until the upshifts are crisp with no hesitation. This should give you clean upshifts. If not, adjust the small screw near the dropout so that the der rotates forward and gives more chain wrap.

knobster

I'll give this a shot. I think I remember reading someone using a little WD-40 in the shifter to sort things out.

It just misses completely. No click, no catch, just swings all the way with no shift. It might catch the second time, may not. If I flick the shifter with my finger, it usually catches then. The up shift works just fine.

Al1943

Yeah, try the WD40 trick, use lots. My 1998 Dura-Ace shifters had the same problem as what you have described. I shot them full of WD40 and they've worked perfectly since. That was 9 years ago.
New shift cables and cable housings may also help the shifting performance. The short housing at the rear derailleur often becomes dirty causing excess friction that can foul shifting. BTW, the smaller shift lever in the back is an up shift, not a down shift.

knobster

I did try the WD-40. It missed immediately before and sprayed it pretty heavily. Hasn't missed since, but I only tried for about 10 minutes and the bike was hanging in the garage. I'll test it out tomorrow.

Thanks for the help guys.