Hi Neil, I presume you've had a new chain too when the cassette was changed? Transmissions bed in to suit, and if the chain as worn beyond a useful life, it will not mesh properly with new parts. Also is the chain set worn/original too?. If there's 'hooked' shape to the teeth, this also needs replacing. 

Hi Neil, I'd check the frame near the rear axel for a small hair line crack which might be evident by a small crinkle in the paint work. I hope you don't find one!

Hi Neil,

What make are the downtube shifters? As you may be using Shimano rear gear on a Campagnolo cassette the shifting problems may be due to non-compatibility issues. Try a Google search on 'Shimergo' or check Sheldon Brown's website for more information. It may also be a 8/9/10 speed chain issue as they are all different and don't get on with the wrong components.

If it's not that, it may be a chain wear issue as Kevin says.

I've always been told the order of transmission replacement is chain/rear cassette/chain ring as that's the order they wear in because of the strains of the job they do. On a tandem this will be amplified.

Chains should be cleaned on a regular basis and I've never used the 'on the bike' chain cleaning contraptions. I remove the chain with a rivet extractor and wash it in White Spirit, swishing it around in an enamelled bowl to remove the grinding paste which road dust creates. Tip the used spirit back into a container, wipe out the bowl and then rinse/repeat. Then several wipes dry of the chain pulling it through a tightly held rag. When it's dry, a drop of oil on the roller of each link once it's back on the bike, having cleaned the chain rings and cassette. This has worked for me for the past 50 years (Eek!)

I keep the White Spirit in an old bleach bottle using a funnel to replace it. After a couple of years the bleach bottle is replaced - when the black gunk in the bottom gets too thick. . .

I hope this helps

This should explain the Shimergo issue

https://www.cyclinguk.org/cyclists-library/components/transmission-gears/derailleur-gears/shimergo 

Alan

Hi Neil

"skips gears"....if it jumps onto the next cog perhaps the FRICTION downtube lever needs tightening....but if you have INEXED levers there could be several issues.

We must assume the cogs AND chain are new. Mixing old and new does not work.

"inner front ring won't engage"...adjust the front changer so that it throws the chain further over. (a "chain catcher" would prevent it dropping down behind the inner ring)...or...if the chain drops down but does not engage sounds like a worn out inner ring...also....don't try changing when climbing a 1:4 hill!!!

Notes on mixing Campag & Shimano.

Campag freehubs have different splines to Shimano....Campag cogs are spaced differently to Shimano...You CAN buy special cassettes that are Shimano spaced but fit Campag freehubs....Indexed levers are speed & make specific. Using a Campag indexed lever to shift a chain with a Shimano rear derailier won't work The "shift ratios" are different. (Google Chris Juden's article from a few years ago)

Chainrings...the only difference is that new 10speed+ rings have thinner teeth than old rings. The make is not important. Some chainsets will require "chainring spacers" if the rings are too close together. You get "chain suck" when using a 9,10,11sp chain with an old ring.

 

Good Luck, Chris Peachey