I keep looking at your diagram and the pics trying to figure it out. Just guessing here but, It looks like it might be some sort of pressure relief bypass? That piece you are holding in the pic, does the black part turn, like a regulator? Weird.

On a boat with water injected over-the-transom (OTT) headers, plumbing usually goes: off the jet you’ll have a shut off valve, gate or ball, to a tee and that tee feeds the front of the block the 2 water pump inlets, water circulates thru motor, comes out 2 hoses connected to tstat housing, 1 goes out thru the hull dumping cooling water(dump line), the other line feeds a Bassett tee valve that tees to the header injection. The Bassett tee valve has a ball and spring inside to restrict water to the headers at idle. As you accelerate, the jet increases the pressure in the cooling lines and that pushes the ball and spring allowing water to be injected into the headers.
That’s the basic routing for injected headers. There are valves and regulators that can be added to build or relieve pressure.


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There are a lot of threads about the “proper” way to plumb a jet boat, but that is the basic way.

Just found this
http://www.socaljetboats.com/engine-mechanical-electrical/water-injected-vs-water-jacketed/

The hoses going to the collectors are to keep the exhaust hose cool. Makes a little more sense now.


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Thank you very much for the helpful information! I think you are right, that its a pressure relief bypass, but I'm not sure what purpose it is serving at that location. I was also thinking there should be some sort of valve that limits water at idle.  I think the current system setup I have now runs water through headers whenever the engine is running. I was trying to understand how to cool the block while also have a line to the headers that can be turned on/off, so thanks.  I found that thread that you linked too as well.  Reading the responses on there they make it seem like its ok to run mid-riser headers in a closed compartment (my current set up)? 

       "if budget is what you are looking for then look for a set of midrisers! better then logs, and cheaper then headers! water injected are just like over
        transom headers...they have a jet so to speak at the bottom of the headers that mist water into the pipes! also water injected thru transom have a water
        port near the collector to keep the rubber hose that goes to your transom from burning up..."

if that is an enclosed engine compartment and those headers are not jacketed the first thing you need to do is replace those... they get extremely hot and will catch the boat on fire...

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Yes, the back section of the boat is covered. Thats good to know. Supposedly previous owner has run it awhile like this.  So even with water being injected they're too hot?  Sorry for my ignorance.  Last thing I want is a fire.

Hello all! I just picked up a 1984 Crusader day cruiser and excited to enjoy it with my family.  I grew up with a direct drive ski boat but have no experience with a jet. The boat has a 454 BBC and Berkeley Jet 12JC-A.   I'm going through everything and trying to learn all I can while getting it ready to take out. I'v been reading and searching a lot but have a couple questions I cant quite figure out.  I'm trying to figure out the plumbing for the block and water injected exhaust.

What I see ( and could be missing something) is water coming from jet, splitting and having water go into the block, the other split goes to some sort of valve or regulator(which currently has an opening??) and then to the end of the headers. The water leaves the block and then goes to the headers to be injected.

Is this a correct set up? Should there be something regulating amount of water being injected to headers? And any ideas what the valve/regulator with an open end is?  Any help is GREATLY appreciated.  I'll try and post pictures below and a diagram I drew.