I have a Volvo Penta 4.3L v6 inboard motor. I'm suspecting it might be time to change the risers and manifolds. The last time they were replaced was Jan 2012. Last season and this season when draining the exhaust manifold there was plenty of metal scale that came out with the water. I had to keep poking my finger in the drain hole to keep getting the scale pieces out. I pulled the port side, just to inspect, they look OK but not pristine by any means. Here are a couple pics. Any advise is greatly appreciated.