Chain jump

If you have the time, post 2-or-3 pictures (e.g., side view, rear view, etc.) which show how your rear derailleur looks when the chain is on the smallest cog & largest chainring ...

• You may have too much slack in your chain ... post a pic when the chain is on the smallest chainring AND the smallest cog, too.
  
• Your indexing may be OFF the mark whereby the chain is not nesting properly on the teeth of the particular cog you want it to be on ... BTW/IMO, it is easier to set the High-Low stops on the rear derailleur when the chain is not installed because there is little left to the imagination as to whether-or-not the pulley wheel is properly aligned with each cog.
  
• the chain (if not new) could be worn/stretched
  
• the cogs (if not new) could be worn
  

I am sure that there are many other avenues to investigate ...

Regardless, you may be better off if the chain is not on the smallest cog when starting off from a dead stop ...

OR, if you don't need-or-want to shift, you may want to consider setting your bike up as a Single Speed.

I agree with alfeng and at the age I would suspect a combination of a worn chain and cog. Most Bike shops will check the chain strech for free and it takes anly a few seconds.

Quoted post said:

Originally Posted by Wrench monkey [IMG]/img/forum/go_quote.gif[/IMG]

Hi new here figured I'd give this one a try, seems like mtbr is populated by beamer driving self obsessed snobs. Anyway I have a 21 spd trek 4900 that I just built, problem is when chain is on 7 small ring rear and 3 large front the chain skips 1 or 2 teeth on the rear sprocket when I start off. I adjusted the rear derailer forward to allow more links to contact teeth but this did not help.

Nothing against BMW or owners just tired of self-absorbed a$$holes.

How do you tell the difference between a BMW and a porcupine? Generally the porcupine has the [censored] on the outside.

It could also have something to do with the cable being too tight or the limit screws on the derailleur set wrong.