replace spark plugs but make sure you use AC delco from a current Mercruiser catalog. number has been superceded and there were alot of problems with the denso spark plugs. Im surprised you didnt store a misfire code in freeze frame though. Did you check freeze frame or only current Faults?

bad boost fuel pump will do that....swap pumps and see if the other motor does it...replace..

I did check store faults and there was none. hp500 I was thinking the same thing because when I checked the vacuum that it would pull it fell a little short of the spec.

if your fuel pressure is good WHILE EXPERIENCING THE POPPING (not just on the gauge idling), then look at ignition.

Yes, broken springs will cause popping too but a weak ignition will as well.

just curious on what was the actual problem and what
resolved it. So we can store it in our mental databases

Flappers?
 

No way it could be the exhaust flappers could it? Sometimes they pop like this on my old 525 SC.....

Well we will never know the the boat is now on the bottom. We just whent thrue Ike and its not good. My shop is ok but the marina is in bad shape.

Sorry to hear that but glad your shop is ok
good luck joe