1975 351cu 233hp Mercruiser, riser block off plates or not?

[soulsalvation]

Sorry Chris. I meant to say a disconnected front manifold tube.

 

[soulsalvation]

I forgot to mention. When the boat came back from the mechanic only one side had been done which was the side with the burnt riser bellow so the other side still had the plate and when I ran the engine it was obvious the side that still had the plate heated faster and was significantly hotter.

 

[soulsalvation]

So the end result of the water test on my exhaust cooling system is this: boat was tilted up as if in bow up attitude and I ran the engine at 1000 rpm disconnected a return tube from the manifold and there was a lot of pressured water returning to the thermostat housing and all lot of water flowing out of the lower exterior transom. This to me is a positive result and next will be a careful lake test. Thanks to everyone’s feedback. Pura Vida to all.

 

[johnkom]

Any more to report?? Hot spots, cold spots? My 233 manifolds run a consistent 140 -150 degrees. Manifolds and risers.

JK

 

[soulsalvation]

actually there will be! for those who may be interested in the end result and not irritated by it. your experience seems to be without doubt but mine isn’t, there are strange things at play here and i’m trying to figure it out. thanks for your reply

 

[soulsalvation]

After much deliberation and consternation the end result of an all out trial with removal of the block off plates was positive. Overall the manifolds ran warm but not hot and the engine about 140 due to the thermostat which should be 160 but what’s 20 degrees. The waters near Montreal in Canada are warm during the summer. Boat ran beautifully and even at a hard run those manifolds never passed 150. That was not the case before I removed the block off plates so to wrap this all up I would say “what the heck are those plates good for?”
Maybe this is the end of fried manifold boots! I could never touch my risers when the engine ran hard; now I can. That’s all folks!

 

[MichaelBC]

I would say “what the heck are those plates good for?”

I was always told they are very important. When you stop quickly the wave at the stern rolls at the back of your boat and and can press water in the exhaust and into your engine. To prevent this the flaps close then water comes in.

 

[Scott06]

After much deliberation and consternation the end result of an all out trial with removal of the block off plates was positive. Overall the manifolds ran warm but not hot and the engine about 140 due to the thermostat which should be 160 but what’s 20 degrees. The waters near Montreal in Canada are warm during the summer. Boat ran beautifully and even at a hard run those manifolds never passed 150. That was not the case before I removed the block off plates so to wrap this all up I would say “what the heck are those plates good for?”
Maybe this is the end of fried manifold boots! I could never touch my risers when the engine ran hard; now I can. That’s all folks!

by block off plate do you mean the restricor gaskets between the manifold and elbow or a stainless plate between them?

 

[crazy charlie]

I was always told they are very important. When you stop quickly the wave at the stern rolls at the back of your boat and and can press water in the exhaust and into your engine. To prevent this the flaps close then water comes in.

You are referring to exhaust flappers.SSal is talking about something between the manifold and the riser.Charlie

 

[poconojoe]

Wasn't this already discussed?
Why start another thread?

Didn't @achris already explain the purpose and need for those plates? I consider Chris the ultimate expert on Mercruisers due to his abundance of experience, knowledge and great videos too. I wouldn't doubt his explanation.

I understand how you can arrive at this conclusion, but why aren't thousands and thousands of us with I/O's having the same problem?
I suspect something else is going on.

Just saying...

It's your boat, you do what you want.

 

[Scott Danforth]

yes, was answered here https://forums.iboats.com/threads/1...-block-off-plates-or-not.748366/#post-5689650

 

[GA_Boater]

New thread has been merged with the original thread. Hope I picked up all the new replies.