reducedfatoreo

I've got a set of Shimano 600 6401 downtube shifters on my Cannondale '85 ST 500, shifting a more modern Tiagra FD 4403 triple that came off a 2002 R600 CAAD4.

While spinning everything runs very smoothly, but when I start to mash for a sprint or stand up to take a short hill in the big chain ring, the shifter starts slipping towards the middle chain ring. It's never gotten so bad that it actually downshifts, but there will be a substantial amount of chain rub, and I always have to reach back down and push the shifter back into place to stop the rub, for which I have to sit back down or lose some momentum, of course.

The shifter is screwed in by hand, but is very tight. The derailleur is adjusted as it ought to be. My guess is that the shifter is slipping a bit whenever there's side-to-side flex from putting a lot of pressure on one side of the bike then the other. But how to stop the slip? Is there a gasket in the 6401 that might be worn down?

Any mechanical advice would be much appreciated. Thanks!

clubman 

C-dales don't flex much so I'd suspect the shifter is the problem. I've seen a rear 6400 shifter with cracked internals, maybe you should take a look inside?

CardiacKid

It is possible that the cable is hanging up where it goes under the bottom bracket. Clean and lube that area. That is the quickest and easiest potential cause to fix. If that doesn't fix the problem you haven't wasted much time or effort.

IthaDan 

Do you ride in baggy shorts?

rccardr 

I wondered the same thing.

If not, check to see if there's a small copper washer on the inside of the shifter. It may be worn too thin. Alternatively, you may have something caught in there between the shifter and the downtube adapter that makes it impossible to really seat and tighten the lever.

John E

You can always try to increase the compression by stacking another washer under the mounting bolt.

cb400bill

I had this same issue with a Suntour Cyclone shifter. Eventually, I just swapped the shifter for another one.

Chrome Molly

Strange that the shifter actually slips when the BB/crank moves sideways under mashing. I'm inclined to think that maybe the cable is hung up under the BB somehow, or doesn't run very easily (maybe lubricate it under the BB shell). I'd think that the FD would spring back once you're done mashing if the cable weren't hung up between the FD and shifter, as the FD is the part that should move of the two when normally opposed.

onespeedbiker

I've had the same thing happen and it would down shift. I'm not sure the cause, but slight tightening of the preload screw stopped it.

dddd

I'm not sure if this applies to 6400-series left shift levers, but Dura-Ace 7400 left shifters had a helper spring inside that needed to be somewhat "wound up" during installation, to help counter the cable tension from the front mech's return spring.

Perhaps the 6400 shifters do too, and if so, remember that these shifters were designed to operate doubles, not triples.
A triple's increased lever throw might leave the lever with no assist from the helper spring when in the big ring.
This is doubly significant when you consider that many of Shimano's similar (and similar vintage) front shift levers had no adjustability in terms of friction level, the friction level being pre-set by force from a spring washer of some sort (with the mounting bolt tension bottoming on the end of the bushing, not on the washer/lever sandwich.
So, combining a loss of helper-spring assist with a pre-set friction level might very well result in a degree of failure of the front shifter to hold the top-gear position under the stress of (albeit minimal) frame flex.

gaucho777 

In my experience, ghost shifting is almost always a problem with the shifter. In almost all cases, it can be eliminated by either tightening the shifter mounting bolt and/or cleaning of the shifter assembly. I avoid putting any oil in the shifter assembly unless it's sticking. If you absolutely must put a lubricant in there, do so minimally, wiping away any excess so as to leave just a very light film on the washer.

+1 to John E, adding another washer if needed for added tension.

eschlwc

overhaul the whole mechanism and use plenty of lube. the one time that didn't work for me, i used just a tiny bit of blue locktight on the threads, and then finally changed out that non working lever for vintage campy.

Phil_gretz

Me, too, although the problem was on the rear derailleur side. I wound up changing them out for SunTour ratcheting Power Shifters. Problem solved...

dddd

In my experience, these types of Shimano shifters, both front and rear, are not conducive to rebuilding.
As I said, the friction level is pre-set, non-adjustable, and imo not serviceable.

In general, regular friction shifters work somewhat poorly if not lubricated in both the pivot and friction surfaces.
Freshly-lubricated shifters that slip will often "break in" to perform perfectly if given time for the fresh lubricant to become contaminated with enough surface-wear material so as to grip as intended.

bradtx

reducedfatoreo, Here is a set of instructions for the 105 light action DT shifter (disregard the 400 shifter), which works the same as the 600 and D-A shifters. It is important that the lever at rest is slightly raised as shown and that the spring is not broken.

Brad

https://techdocs.shimano.com/media/te...9830612533.pdf

PS I've used these levers with a triple and they worked as expected.

reducedfatoreo

Awesome, thanks so much for all the replies, everybody. It's not a big deal, obviously, but this bike is due for its yearly maintenance anyway, so once I get it in the stand I'll be sure to follow all the advice here.

My guess is that it is due to the internals since the mounting bolt is as tight as it can (safely) be, and the cable run, including under the BB, is nice and clean with no hangups.

I have another 6400 series shifter, so I might just be able to swap out the washers and see if that fixes the issue. bradtx, thanks for the tech doc, that should come in handy!