My bud had a warranty claim on his and the merc rep made them put the boat in the water to see how high the risers are above the water line. All stock stuff from wellcraft , they found them to be high eonugh but had me worried thinking about that for a minute. He has silent choice and I have throught the hub only. Never a problem here. Did replace the flappers when I had the motors out last season, old style had a tendancy to split at the hinge pin that runs through them and get stuck in the bottom of the y pipe. How old are the manifilds and risers? They may be suspect.

when the new engines were put in there were new manifolds and risers put on.

Pressure check them again to be safe. One bad winterization can split that exhaust real fast. Mud blockages and such. ( Do they use antifreeze?)