Bilge siphon system?
I'm looking at winterize procedures for another member and I came across a chapter on the "bilge siphon feature" of the M-2 boats.
This system works like the bailers of a jet ski. You have a hose that is connected to the pump nozzle. The tube comes into the engine compartment and is routed to a clamp at the exhaust manifold to create a loop seal to keep the water from running back in once the boat is off plane.
This system is designed to work when the engine exceeds 3000 rpm.
If this hose is still attached but laying in the bilge of the engine compartment, then the water can flow in easily.
If it's hooked up correctly but you have electric bilge pumps, make sure this system is dis-connected and plugged.
To identify it, look for a clamp and hose on the exhaust manifold. If it's not being used, the manual states that it is coiled up at the clamp of the exhaust manifold. If it has been used or is in use, the end of the hose will be laying in the bilge and have some type strainer screen.
I was kinda of surprised when I ran across this chapter. Even more surprised to see a boat use this type of "vacuum" system. This system is used in ski's which is logical, there isn't much area to pump out, but in a boat?.....I'd have never thought it would be there.
There is also another option for the Mercury powered boats, that I was unaware of (finding info in the maintenance section. I am normally in the engine section). It's called the "water by-pass system" and is used to improve powerhead cooling at idle speed. This is an option; not something connected by the dealer. If someone has installed it by the manuals instructions, there will be a thru-hull fitting in the transom of the boat, 2" above the water line. If you have this connection, make sure it is connected. If it's not used, it is suppose to be coiled up and strapped in the engine compartment.
But with your problem occuring at high rpm, I'm leaning on the first mentioned problems, leaks of the cooling system or the bilge siphon system.
I hope you have enough information now to find and fix this problem. Let me know what you find out!............:cheers: