Mercury 4.3 running rough - ideas?

[Lou C]

If you do wind up removing the elbows you can look inside the center exhaust passage for rust trails. There should be no rust there only black carbon. If you remove the manifolds you can see into the exhaust ports to determine if the exhaust valves are getting corroded. If so it’s time for a valve job. If just water causing rough running then replace the exhaust and count your blessings. Merc OEM is pretty pricey for the dry joint units but Barr Marine makes good quality aftermarket exhaust, I have theirs on my old OMC.