JayhawKen

When I am in the big ring I cannot upshift to the last cog on the cassette (the 12T). Click the shifter, but the chain does not move down to the last gear.

When I am in the middle ring it shifts instantly to the 12T (and all the others for that matter). Other than this problem, the system works absolutely flawlessly - crisp, instantaneous shifts, almost silent in every gear and I have never dropped a chain.

The patient is a 105 triple with 30/39/50 and 12-25, running a SRAM chain.

So what would cause this very odd problem? I am hesitant to back-off the the lower limit screw since I can engage the last cog in the 39 ring, and I do NOT want to risk throwing a chain into the gap between the last cog and the frame and locking up the rear wheel at 40mph.

DOS

My best guess is that the cable was attached to derailleur when the shifter wasn't in the higest gear. With chain on big ring, disconnect cable from the RD and turn the cranks; if the limit screw is correct, the chain will drop to the smallest ring; click the shifter until it is set on highest gear (i.e. the gear that should position the derailleur under the 12T cog) . Eye-ball the RD pulleys to ensure they are lined up directly under the 12T cog; if they aren't lined up, turn the hi gear screw in or out to move pulleys under 12T cog. Pull cable taught and reattach cable to the derailleur; use barrel adjuster to dial in shifting.

Grand Bois

That only happens with fixed gear drivetrains. You'll just coast.