Failed exhaust flappers

Trevthefox

Petty Officer 3rd Class
Joined
Sep 25, 2018
Messages
97
Hey everyone, I have a 89 460 7.5L King Cobra. Was just doing some general maintenance taking off the risers and manifolds when I noticed that one exhaust flapper was broken. The little rod moved from its bushing and dug itself into the inner wall of the y pipe. The other flapper on the other side was fine but kind of stuck open. So basically they're not doing their job. Luckily no damage occurred.

Rather than replacing with the OEM part, does anyone see anything wrong but removing the y pipe flappers and adding a universal exhaust flapper to where the exhaust bellow clamps on? I'm thinking I could just cut that exhaust bellow and add on the universal flapper to the outlet pipe from the transom. The only difference I see is no water will pass through the outdrive and through the prop on its way out.

I was looking at one of these:
 

dubs283

Vice Admiral
Joined
Jul 27, 2005
Messages
5,231
My guess is it may work okay if you can find the right size and it doesn't interfere with any of the other transom assy components

Caveat may be louder exhaust noise and chance of sooting the transom

You could also abate the flappers, keep the bellows in place and operate the boat knowing the risk of water entering the exhaust. No saying how long you've gone with the current flappers compromised without fail anyways.

Big block V-8's tend to have enough exhaust pressure to keep water out, even at idle speeds. Just need to be cautious of waves hitting the transom when moored/engine not running
 

Trevthefox

Petty Officer 3rd Class
Joined
Sep 25, 2018
Messages
97
My guess is it may work okay if you can find the right size and it doesn't interfere with any of the other transom assy components

Caveat may be louder exhaust noise and chance of sooting the transom

You could also abate the flappers, keep the bellows in place and operate the boat knowing the risk of water entering the exhaust. No saying how long you've gone with the current flappers compromised without fail anyways.

Big block V-8's tend to have enough exhaust pressure to keep water out, even at idle speeds. Just need to be cautious of waves hitting the transom when moored/engine not running
Thanks for the input. I do want to have some sort of flappers because I have to go in reverse a lot backing up to lobster nets and sometimes that swell is strong and I could see it going too far up the y pipe.

I just dont see these style of exhaust flapper below the water line and wonder if that would be an issue. i wonder if at idle the exhaust is strong enough to push the flapper open against the water pressure. I guess I'll have to test it out.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,766
The factory did thousands of hours of testing. Why not just use factory parts?
 

Lou C

Supreme Mariner
Joined
Nov 10, 2002
Messages
11,959
There is a grommet that holds the pin in place maybe that melted from an overheat or just rotted out. I’d just keep using the OEM style ones. I’ve never had a water up the exhaust problem with mine using 2 different exhaust styles (OEM batwings & then the later 2 piece style).
 

Trevthefox

Petty Officer 3rd Class
Joined
Sep 25, 2018
Messages
97
There is a grommet that holds the pin in place maybe that melted from an overheat or just rotted out. I’d just keep using the OEM style ones. I’ve never had a water up the exhaust problem with mine using 2 different exhaust styles (OEM batwings & then the later 2 piece style).
The grommet on the starboard side of the ypipe came loose and the pin was driving into the inner wall of the y pipe. I'd have to sand down the damage and do some work with some JB weld to make it fit again. Also the flapper on the starboard side was stuck open. I had to give it a good amount of force to get it loose so it definitely wasn't doing its job.

I replaced both of these flappers about 2 years ago with OMC OEM parts. I was just looking for an alternative since it didn't seem like these were working very well. If i replace with OEM again, I'm going to want to make sure they're still functional, and the only way of checking them is by removing the risers which can be a hassle
 

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Renken2000Classic

Petty Officer 2nd Class
Joined
May 10, 2022
Messages
191
I've got a rigged-up single flapper on the bottom of the pipe that has worked for me for a while. Maybe I've just been "lucky"? Mine's the 4.3L Cobra; don't know if it'd work for yours or not.

I don't recommend it based on the more cautious opinions; I'm just sharing my experience. My boat isn't ever anchored or moored with waves hitting the back of it, but it's had a throttle chop or two. I generally slow it somewhat gradually.

I think a wave would tend to close mine, sort of like the one you pictured.
 

cyclops222

Petty Officer 1st Class
Joined
Mar 21, 2024
Messages
257
Flappers are strictly....Eye Candy. Get the correct height of exhaust risers to stop all risks of back flow into a cylinder and bent connecting rods inside the engine.
REVERSION water flows DO DO happen. I have pulled all the sparkplugs on boats that saved some money with TOO LOW of a risers. On the St Lawrence River during a cruise. On a 3 day holiday cruise. NO mechanics available.
 
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