Didn't Mercruiser have a problem with reversion on the 454 and 502's with stock camshaft and thru-hull exhaust (there is a sevice bulletin on this)? Their recommended solution for this is the exhaust resonators.

Still do not know if these work.

Do a leak test on number 7. You may have an exhaust valve problem as Brownie said.

BT

Check the stainless tip that bolts to the cast iron riser. There should be a dam welded half way up. Iv' seen these rot out and cause the problem you are decribing