Spoke

I have Shimano Ultegra ST-6703 shifters on my bike. The smaller shift lever that moves the chain to a smaller sprocket or chainring occasionally will not “engage” and not move the cable, thus the derailleur and thus the chain to a smaller sprocket of chainring. In other words, occasionally when I push the smaller shift lever inward (toward the center of the handlebars), the lever simply swings in but does not shift the chain. If I push the lever slightly forward while pushing it inward, it seems to work sometime. Is this a known problem? Has anyone else experienced this type of problem? Any suggestion as to what the problem is and how to fix it (short of buying new shifters)? Thanks!

Ride On!
-Spoke

Andrew R Stewart 

Yes, this problem has been known for a while. I have read of differing reasons from the common (especially with flat bat STI lever pods) old grease drying out and becoming gummy/sticky and the resulting ratchet pawl not rotating on it's shaft enough to engage the teeth of the spool to a little spring inside the pod breaking and therefore not holding the large lever in place while the small lever is pushed on.

The old lube problem (which is more common, again) is best fixed by soaking the lever pod in a solvent then blowing out the solvent when things begin to function properly. then replacement lube can be sprayed/dripped back into the pod, often aided by compressed air. Some will just spray a thin lube and work that in but I have found a longer solution is to remove the old lube first. I also relube with a thin oil first followed up by a thicker one.

The broken spring problem can be worked around by just holding the large lever in place while shifting with the small one. This is very awkward and won't usually be done any longer then needed to replace the lever complete.

I wait to read other's take on this. Andy.

Al1943

I would first try shooting lots of WD 40 into both shifters, once with the bike upright and once with the bike upside down. The idea is to loosen the old hardened grease. A second treatment may also be needed.

When you work the smaller shift levers you are releasing cable to the tension pulled by the derailleur spring. Anything along the cables path that may cause increased friction can account for hesitant shifting and even a failure to shift. If your cables and cable housings are 2 or more years old or if they have been exposed to dust or dirty water spray there can be an increase in friction within the cable housings, this is especially true for the short housing at the rear derailleur.

Excess friction can also be found at the hinge pins in the derailleurs if dirty.

Andrew R Stewart 

I had assumed that the OP had already explored the function of the lever with no cable attached as well as the smooth and free function of the rest of the system but it is a good point to repeat, thanks. Andy.

cyccommute 

Another thing to consider is sugar. It seems silly but if you use a sports drink and it drips from the bottle, it can drip down onto the cable guide under the bottom bracket. Once there it can cause real problems with shifting...both up and down. It has happened to me and, just last week, someone came into the co-op with a broken STI shifter. We found a used shifter that worked off the bike but simply wouldn't downshift and had difficulty upshifting as well when installed. I happened to put my hand on the cable guide and my fingers stuck to it. Once we flushed the guide the new shifter worked like a charm.

The old one, however, was broken as the extra force needed to make shifts had biffed the shifter.

SlowAndSlower

Realize this is an older thread but found it in my search to confirm what I found fixing my 6703 left shifter after it jammed up on the large chain ring.

I did the remove and flush with WD-40 to no avail. Looking at the mechanism I noticed the hex screw that appeared to be the main shaft. I tried tightening it and found it was very loose. After tightening the small lever started to working again. I tightened a bit more and reinstalled on the bike and everything seems to working normally.

I been searching to find conformation that this was the problem and found this YouTube video which seems to confirm my solution. I did find some cases where the nut on the screw has actually fell off.

Interestingly enough a rider on the Tour had a very similar problem, perhaps the same.