Bike wont engage 3rd biggest cog for some reason

Raffles

the chain and cassette are shimano 105 series 10 speed and they have just 500 miles on them. The front and rear derailleurs are 105 series also. Problem is that if I change from 2nd biggest cog to 3rd biggest I get the chatterring sounds and the chain jumps from cog 2 to cog 4 without settling on the 3rd.

this doesnt happen on any other possible gear change up or down, any ideas what could be causing this and a remedy ?

BikeSwan

It sounds like the chain is rubbing against the front mech. Try "trimming" the front mech (moving the front mech with the screws on the top of the mech) so that you dont get any sound when that happens. If you dont know how to do that, send it to a bike shop and they should be able to help you out for minimal cost.

Another thing to watch out for is not to "cross" your chain. Dont run the largest cog in the front and the largest cog in the back together because that's an inefficient line for the chain to follow. I find the most efficient way to think of it is to only use the bottom half (Middle to smallest cogs) of the cassette in any given ring other than the granny ring.

Hope that helps!