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Thread: Plumbing Diagram for Mercruiser Manifolds on a Jet Boat

  1. #1
    Does anyone know if it's possible to run the water through stock Merc Manifolds before going to the water pump flanges?
    The way this boat was set up is as follows:
    Pump to ball valve.
    Ball Valve to "T" then into motor through water pump inlets.
    Exits motor through water outlet on intake.
    From water outlet to inlet on the bottom of the Merc. manifolds and out the exhuast.
    I'd like to heat the water up before entering the block. There is a 1" fitting on the front of the riser as well as a blocked in/out on both sides of the manifold next to where the riser mounts.
    I'm not sure whether the two that are blocked are inlets or outlets.

  2. #2
    Whats up dude. Still haven't gone out yet, looks like the maiden voyage will be on thurs @ foxs. I am not quite sure that you want to reverse the cooling. The engineers spend mucho time figuring all that stuff out. Is there any benefit too doing it reverse??????? Someone will post something intelligent, not me though. Anyhow it was nice to meet you.

  3. #3
    It sounds like you will have to separate the exhaust manifold from the riser with a solid gasket first. Right now the water probably flows through some holes or slots in the flange between the manifold and riser. If you block this off, you can plumb in/out of the manifold, through the motor, then out the risers.

  4. #4
    Hey Slick,
    Jets do use reverse cooling.
    plumb from each side of T to bottom of manifold.
    Out of top of manifold at front to water inlets
    at front of block (by h20 pump cover).
    This pre-heats the water through manifolds before entering the block.Water runs through block and out thermosstat housing at front of intake.
    Plumb from t-stat housing out to top of collectors/risers. There should be a gasket between the manifolds and risers.
    Hope this helps,

  5. #5
    That's how I thought it should go. I just didn't know if that was an inlet our outlet on the top of the manifold in front of the riser.
    If I plumb from the water outlet on the manifold to the inlet on the riser, what keeps water from gonig back down the manifold into the block?

  6. #6
    That's what the solid gasket is for...
    And by the way, the manifold doesn't care which holes are inlets vs outlets, the water will fill the cooling jacket and flow through either way. Use whichever is most convenient for your rigging.

  7. #7
    Thanks for the info, guys
    Where does one pick up a solid gasket?

  8. #8
    "New Gasket?"
    Try Eddie Marine; I just changed out the stock manifolds with EMI'S. The Risers look the Same.

  9. #9
    If EMI doesn't have them you might give Larry a call in Tucson. He stocks a lot of stuff or might be able to point you in the right direction. He is also very helpful. 520-623-5373

  10. #10
    Thanks again. I stopped by my jet guys this morning and he told me not to sweat it and to run the way it's always been.
    Whew, less work before OP6.

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