<WARNING>This is for the tensioner on the B12. I just noticed you have a 250. I don't know if they are the same style. Two housing allen bolts with a 19mm or so big bolt in the middle?
You might want to make sure the adjuster is gently 'tensioning' against the chain before the spring and bolt goes on.
Before installation, push the tensioner all the way back 'open', install and use an allen wrench to gently push it (click, click click)in towards the engine until you feel a little tension against the chain. Then you can install the spring and the bolt. Don't jam it in there. That's all.