I had the same problem on new 7120. Dealer mechanic notified CIH tech assist. They told him the valve body that controls this might have a problem. He took it off and all apart and found some rough edges on an internal part of the valve. He removed the scored marks with
real fine grit wet sandpaper, cleaned and flushed out the whole thing. Reassembled everything and low and behold, it worked perfectly. A nice gradual header engagement. I suggest you contact your dealer about the problem. Hope this helps.