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I purchased a bike from Bicycles direct some time around the first of February, and have always had trouble with the left shifter jamming on the down shift. The gear set is current Ultegra STI, i.e. with carbon/plastic shift lever.

The first time it happened was on a damp day when the temperature was near or slightly below freezing. Some combination of fiddling got it shifted, but it jammed again quickly. When I got it inside at home, it started working again.

I have had continuing issues with the mechanism jamming. When I move the down shift paddle to the stop, nothing happens. Usually, I just have to put some pressure on the up shift lever to free the down shift mechanism. I.e., it's like both ratchet pawls are engaged.

An LBShas suggested that the grease might be sticky and the cold weather behavior suggests this as a possibility. As of yet, I haven't tried washing out the lever with something like WD-40 and relubeing.

I’ve used Campy Ergopower up til now, so I don’t have familiarity with STI, and don’t have enough regular interaction with other riders to get feedback from them. I can't believe this is normal behavior, as often as this is happening, or that Shimano would be as widely used if it were. Is this a warranty issue? Wash out with solvent and relube? What with?
 

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that kind of behavior are usually reserved for much older STI shift levers. my 1994 STI levers was acting up real badly in recent years, i finally followed tips found on the forum, by flushing WD40 into the levers, wait for the WD40 to finished dripping, then spray some lithium grease.

I tried to shift without grease, it's too dry and difficult to shift. After the grease, the levers shift flawlessly again.

This is jamming problem is particularly bad when temperature drops below 50F, but after flushing, it works fine at all temp for me. good luck
 

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Have you correctly adjusted limit screws and cable tension? It sounds similar to the behavior when the cable is excessively taught - because the down shift needs a little more pull before it can release.
 

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Burnum Upus Quadricepus
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I tried a pair of 5700 levers. I found them to be especially sensitive to cable issues, and it was worse in winter. Shimano's Dura-Ace cable and housing set relieved much of the problem for me, although shifting was always stiffer than with the older style lever with the "flying" shift cables, especially downshifting to a larger cog. Be aware too, that the D-A cable sets are designed to be installed dry, that is, without Tri-Flow or other lube on the cables or in the housing.

Possibly not your problem, but it seemed to me to be a lever issue too, until I replaced the cables. It's probably a cheaper, easier thing to test before delving into the levers themselves or any warranty issues.

Ultimately, I didn't care for the ergonomics of the new levers (Everything they did to help folks with smaller hands works against my wide hands and long fingers.) So I reverted to the older style lever.
 

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sounds just like what happens when you slightly move the big lever without realizing it and move the small one at the same time. Just a small bit of engagement of the big lever will stop the smaller one from working. It figures this would happen in the cold too. I have the same levers and do that once in a while when hands are numb or the gloves are bulky. It's pretty easy to do without realizing it. Try concentrating on just touching the lever you want to move.

the shop is just BSing you about the cold weather being a direct cause. Maybe if you were riding around the north pole but "slightly below freezing". that's an absurd explanation.
 

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Have you correctly adjusted limit screws and cable tension? It sounds similar to the behavior when the cable is excessively taught - because the down shift needs a little more pull before it can release.
I'm betting on this. The cable is probably over tensioned. It needs to have just a hair of tension when in the small position
 

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Dang, if'd been taught proper I'm sure I would have written "taut" in that reply.
It's OK. We understand that you're a Brit & sometimes have trouble with the English language. :D You can just follow my lead. I never make misteaks.
 
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