Chain reguarly falling off top cog (dura ace)

[aserota]

Im still having problems with the chain falling off the higher cog when shifting up. Unfortunately, the old chain did this a could of times and has maked the fsc cranks (aesthetcially) and utimately the chain snapped.

I have tried adjusting the derallier, but still the chain love to jump off the top gear. Any ideas why?

 

[Tynan]

high low stop adjustment?

 

[briank]

If it's marked the crank then you mean it's coming off the big chainring.
("Cog" is usually taken to refer to the ones at the back.)
To amplify Tynan, if you look straight down on the fron derailleur you'll probably see two small screws sitting side by side. These limit how far the derailleur can move from side to side. Put the chain on the smaller chainring then, using whatever kind of screwdriver or allen key it needs, tighten - clockwise - the screw which is further out from the seat post two full turns. This will prevent the derailleur cage from moving so far when you shift up and puching the chain past the outer chainring.
Two turns might be too much: maybe now the cage won't move far enough to push the chain up onto the big ring, or maybe it will shift ok but the chain will rub against the inside of the front cage when it's heading for a small cog (high gear) at the back.
In these cases, back off the limit screw a little.
Fiddle around a bit and you'll soon get it right.

 

[walker]

failing all that you could have a Bent front or rear Mech, check the rear first

 

[02GF74]

assuming it is falling off at the front i.e. the chain ring on the crank.

there are 3 adjustments.
1. check from looking above the derailuer cage is parallel to the chain ring, if not adjust.
2. looking at it from the side, the lowest outer edge of the derailluer cage needs to be about 1/2 tooth height above the chain ring - again adjust.
3. finally there are two screws - as ^^^ said - whcih limit the side to side swing when the cable is pulled. adjust the top one so that the chain does not fall off.