the sealing between the intake manifold and the head is a viton o-ring.
With the pass of time this peace of rubber hardens and will probably not seal as required.
I've been having some trouble tuning my bike properly. Changed the o-rings and managed to get it tuned just fine. And the bike goes like never before. Wouldn't believe that a piece costing a few euros make such a difference!
Changing them is very easy. For both is about half hour work. No need to remove tank or TBs.