Understanding the water flow of a Yamaha engine. Apparently Yamaha had the idea to have the manifolds and the engine block the same temperature. Still not sure if thats a good thing or a bad. The one thing to understand is the ?plate? below the thermostat housing (Btw the thermostat housing is the same as an old Crusader engine). Once the water inside the block exists the engine at the intake it only goes to the front 2 opening, NOT ALL 4. It then flows through the bottom of the manifolds, up through the bottom part of the risers, out the larger 45 degree fitting and back to the fittings on the back of the ?plate?.
Once it gets back to the thermostat housing the rest operates like any other stern drive engine with the exception of the water exhausting. The water on a Yamaha exhausts from the top of the thermostat housing into the top fitting of the riser and out the exhaust.
(Yes, I have my arrows on the flow through the manifold backwards, oops. It goes in bottom and out the top.)
Knowing that the gasket between the manifold and riser is going to get the full water flow of the engine we will need full open gaskets with no blockage or restriction.
On to the good stuff:
- Remove old Yamaha manifolds and risers (ok that?s a given)
- Since you?re going to reuse your old risers now is a good time to clean them up inside and out. I have heard some boaters soaking them in CLR but I haven?t tried it. You may also want to take this opportunity to have your risers surfaced to get a good, flat surface.
- Make sure all the passages are clear. Remember the large bottom fitting on the riser should be open to the manifold and the smaller top fitting should be open to the exhaust. The 2 chambers are NOT connected.
- Mount the Mercruiser manifolds to the engine.
- This is where the fun part begins. The bolt pattern between the Yamaha risers and the Mercruiser manifolds are not the same, but they?re close. Close enough you can use a die grinder or, if you lucky enough, and end mill to make them match. I had a die grinder with a ?? carbide bit and it only took about 20 mins to do one side. These numbers aren?t exact but the bolt pattern on the Mercruiser is a small amount narrower and about a ? inch shorter. Remember to take it out of both sides or maybe a little more from the front as is easier to get to when you bolt them back on. I also took a little out of the walls of the riser and cleaned up some of the surfaces. I tried not to take too much but did take a little more from the bottom side since this is where I would need to get the box end of the wrench onto the bolt head.
- Once you have the open gaskets (did I mention you need OPEN gaskets?) and using every brand of socket and wrench you have, bolt the risers onto the manifolds. Some wrenches and sockets have thinner walls than others.
- Now at this point the solenoids for the trim pump and linkage plate will bolt right up to the Yamaha risers with no problems.
- The next modification we need to do is the fuel filter bracket. The Yamahas had 2 bolts holding the bracket on. Well I figure that a 5/16[SUP]th[/SUP] bolt can hold a little fuel filter and a bracket. You will need to drill a hole in the fuel filter bracket and the bracket for the plugs. As for fuel filter bracket with the fuel lines connected, so you don?t have to make any modifications to them, mark the spot close enough to the hole in the manifold. I pulled the bracket out a little so I could see behind it and got a close measurement. The new hole will be closer to the engine and about centered of the old holes.
- For the plug bracket I just drilled the hole so the plug bracket was at the same height as it was. Then flatten out the bend closest to the plug to get the clearance you need for the back of the plugs.
- The last modification is the probably simplest. The lower manifold hose connections. Install the drain elbows onto the bottom of the manifold. On the port side cut the old hose to fit the location of the new elbow. On the starboard side you will need to cut off the part that expands from 1? to 1 ?? and attach the 1? to 1? barbed coupling. Add enough 1? heater hose to reach the new drain elbow.
