One not unlikely cause of similar symptoms can be simply a tight link in the chain. This is especially possible when this happens immediately after assembling a chain. I'd detension the chain and make sure every link flows over a falling curve with no binding, it might even be easier to remove the chain to perform this check. It will most likely be the point where you joined the chain last. This can be rectified by simply grabbing the chain on either side of the sticky link and applying a pretty vigorous side force to the chain back and forth until the link "flows". Whenever replacing a chain rivet I recommend checking it for free pivoting, it only takes a moment and can sometimes avoid later hassle.
Kurt Sperry Bellingham, Washington USA
On Fri, Jan 1, 2010 at 7:44 AM, earle.young tds.net <earle.young@tds.net> wrote:
> John and list,
>
> Happy New Year to all.
>
> John asked, "When riding with much pressure on the pedals, the chain
> "jumps", and what I mean by jumps, is that it crosses the top of the
> freewheel gears. The freewheel is a Campagnolo Ergal with excellent teeth
> and the chain is a new Everest slotted lightweight chain."
>
> Chain skip such as you describe is caused by a mis-match of chain and cog.
> Even though your freewhwheel looks like it is in good condition, it is worn
> enough that a chain roller is hitting the top of a cog instead of engaging
> behind it. This will happen most frequently on the smaller cogs of a
> freewheel, especially 13 or 12-tooth cogs, and even more commonly with alloy
> cogs instead of steel cogs. Alloy freewheels wear very quickly, and are
> considered special event equipment, even for professionals.
>
> Unfortunately, there is not much you can do to correct this except to get
> new cogs or freewheel.
>
> Sorry to be the bearer of bad news.
>
> Earle Young
> Madison, Wisconsin
> Offering expert wheelbuilding services for classic and modern bicycles