1979 Mercruiser Manifold Conversion

[stewsimac]

After having a log style exhaust manifold crack I got a set of center riser manifolds and risers for the 5L (305) chevy in my boat. It is a fresh water cooling system. I hooked the hoses up the same way they were connected on the old manifolds and used the same riser to manifold block off plate along with the old thermostat housing. I now have a problem that as I increase speed the engine heats up and if I decrease speed it cools down. Seems like a lack of flow to me. Should I change the riser to manifold block off to a restricted type or should I change to newer style thermostat housing? There doesn't seem to be any information on the correct mix of parts when doing this conversion.

 

[Bondo]

It is a fresh water cooling system.

Ayuh,.... Welcome Aboard,.... Yer sayin' ya got a heat exchanger, 'n run antifreeze coolant,..??

 

[alldodge]

I'm guessing its either with your thermostat housing or a restriction between the drive and thermostat.

Got a pic of the stat housing and hoses?

 

[achris]

Do you have flow diagrams for both style cooling systems?

 

[wahlejim]

Are you using the same hoses? My '83 305 has preformed hoses for the log style cooling system. If you tried to manipulate those to fit the center style, I would imagine you would be kinking the hoses.

 

[NHGuy]

So is it cooled by outside water or heat exchanger and antifreeze.

 

[stewsimac]

OK here are answers to the questions above:
Bondo - I should have said "raw water cooling system".
Achris - I have several diagrams but non mention what to use between the riser and the manifold.
wahlejim - I have used a couple of the old hoses that fit without kinking and used new ones on the other runs.
AllDodge - I will see if I can add a photo here ! ! !

 

[stewsimac]

NHGuy - It is cooled by fresh water from the lake. No heat exchanger or antifreeze.

 

[alldodge]

OK here are answers to the questions above:
Bondo - I should have said "raw water cooling system".
Achris - I have several diagrams but non mention what to use between the riser and the manifold.
wahlejim - I have used a couple of the old hoses that fit without kinking and used new ones on the other runs.
AllDodge - I will see if I can add a photo here ! ! !

Yep its the hose routing (to many hoses) and need to change the thermostat housing.

From what I see you started with this

And this is what you need to change to


Suggest changing the thermostat housing, remove the other hoses going to top of manifold, and also to risers. Then make sure to have feed thru gaskets

 

[stewsimac]

AllDodge - Right, thanks. I figured that was it but needed some conformation before getting another thermostat housing.
I will let you know how it goes when I am done.

 

[achris]

Your original setup was a 'warm manifold' system. A few years ago I did all the drawings for someone to convert 'end riser' manifolds to centre riser.

Looking at your picture you have water going into the madifold at the top and out at the bottom. You need to switch those hoses (probably easiest at the thermostat housing end). You also need a 'restrictive' plate bewteen the manifold and the elbow (has a small, about 3/16" hole in it for manifold air to bleed off into the elbow).

Alldodges diagram converts the system to a 'cold manifold' set up. You'll need a different thermostat housing.

Chris..........

 

[stewsimac]

achris - Thanks. I will try that before I do an "expensive" thermostat housing change.

 

[mkulin]

Quick question what is the gasket arrangement on the starboard riser to manifold on the original log setup?

 

[achris]

Gasket and seperation plates between manifold and elbow is a 'restrictor' plate. Closes the water passages, but with a small (about 3/16") air bleed hole.

 

[stewsimac]

mkulin - both gaskets were as described by achris above. There was a stainless plate between 2 gaskets.

 

[mkulin]

Both sides right?

 

[achris]

Both sides right?

yep

 

[stewsimac]

Well it looks like it will be spring before I get at this again. Too many things happening right now. I will finish this post then. thanks for all the help.

 

[stewsimac]

Finally finished the conversion to high rise manifolds. Works great ! ! ! Thanks for all the help.