Out drives have flappers in the exhaust pipes behind the engines.

Inboards do not normally have internal flaps.
Some boats have external flaps on the exhaust ports at the transom. To prevent water ingress when backing down hard.

Do you have thru the bottom exhaust?

 

I thought the same until I spoke with a Mercruiser rep today about an issue. He thinks they are in the log. I'm running a scope in the exhaust this weekend. We will find out what's in the mysterious log.

 

I didn't comment earlier because I don't know the routing or contents of the V-Drive exhaust system. IE. if it does or doesn't have mufflers by design and/or if the exit is through the transom or through the bottom of the hull.

In any case, in the stern drive editions of a 7.4 the flappers are mounted in the TOP of the Y-pipe, below the elbow that connects the horizontal log to the Y-pipe. By nature of design they appear to need be in a vertical run of the exhaust. Additionally, the Y-pipe is notched to accept the rod on which the flappers pivot.

In either case, if this arrangement is similar, the flapper(s) would be located here:

Again, being unfamiliar with that particular exhaust, I don't even know what that down pipe in the picture is connected to. If it has flappers though, that is where I'd expect to find them.

Mine is currently apart so if I can beat the storm out there, I'll go uncover it and get a pic or two so if my explanation is lacking it will make more sense.

 

I didn't comment earlier because I don't know the routing or contents of the V-Drive exhaust system. IE. if it does or doesn't have mufflers by design and/or if the exit is through the transom or through the bottom of the hull.

In any case, in the stern drive editions of a 7.4 the flappers are mounted in the TOP of the Y-pipe, below the elbow that connects the horizontal log to the Y-pipe. By nature of design they appear to need be in a vertical run of the exhaust. Additionally, the Y-pipe is notched to accept the rod on which the flappers pivot.

In either case, if this arrangement is similar, the flapper(s) would be located here:
View attachment 162011

Again, being unfamiliar with that particular exhaust, I don't even know what that down pipe in the picture is connected to. If it has flappers though, that is where I'd expect to find them.

Mine is currently apart so if I can beat the storm out there, I'll go uncover it and get a pic or two so if my explanation is lacking it will make more sense.

Ignore the yellow circle below but this is the best photo I have. The exhaust splits above this photo to be both thru-hull above the waterline on the side of the boat and then under the water, directly below the tube. I guess I don't see how a flapper could be used in this application but I've been wrong many times before

 

Has anyone ever had an issue with them being stuck? I think this design may have been used on other models as well.

I'd say that the likelihood of them getting stuck is pretty darn thin. What would be the more common occurrence would be that they 'break' and come apart such that one side or the other gets caught downstream and create a blockage that can wreak havoc.

When I did the starboard side on mine, the flapper was torn about half way across the pivot point so it was on it's way out. The port side (which is currently apart) had not yet suffered that affliction.

I don't believe this to be a common problem, but then again, the flappers should be changed/replaced along with the manifolds. A man should not expect them to last across multiple manifold replacements.

 

Thanks for the pic, now I have a better idea of what y'all are dealing with.
With that arrangement, I could see the need for flappers being greatly diminished if not eliminated.
Out of curiosity, are those down tubes aluminum or are they glorified composite resin?

 

I did find out. There are no flaps in this particular "log", only 2 tube's with varying depths inside the muffler. In my case, the water level inside the muffler (added to check for blockage) was high enough that the above water port was somewhat blocked, causing the exhaust pressure to relieve to the underwater port. After running the boat for a 8 hour trip, the issue resolved itself.
All discovered with the help of Centek, the muffler manufacturer.

Did you find out what is in the log?