Hi Mark,
Do you mean when the tensioner is on the chain - it makes the chain too tight ? as the spring should only take up the slack.
It might be that the chain you have is just a little to loose but not long enough if you know what I mean.
I have ordered a brand chain (and horn)
. I think that will sort all my problems out.
I would not mod the C5 to fit the chain, I would put some links in the chain to fit the C5 or buy a chain. As karl has said the rear axel is hollow and can bend under stress if the chain is to tight. Not worth the risk IMO.