"Hard to say on the HP...but m
"Hard to say on the HP...but my guess is 240. Unless it has GT40 heads...then more like 275-280.
The pressure from the hose forces water through the cooling system, then out the manifolds and up and through the risers.
Some risers allow this water to flow backward into the manifold where it then enters the exhaust valve in into the cyl. This is not necessarily a good thing for the engine.
Certain manufacturers designs allow this to happen. It doesn't on all. But, you don't have any way to know and the penalty for error is massive.
Solution: Don't hook up a hose. Rather suck cooling water from a drywall bucket into which you have jammed the hose. You will need to jury rig a short length of hose and a T fitting to do this. The benefit is that you can easily flush the system without danger, and you can easily add antifreeze when you go to winterize. The whole deal can be done for less than 5 bucks.
You can use a hose if you start the engine first and then turn on the water a second after the engine catches. Just prior to killing the engine, turn off the hose. That water pump impeller will fry itself in short order if you allow it to spin with no water. Don't ask me how I know this.
Since yours is an I/O, it's going to be a bit trickier as the raw water pick up pump in the lower unit is spinning. You may have to use the muffs. Just don't let the hose run with the engine off unless you that your risers do not permit back flow. I don't have this issue as mine is a straight IB. Somebody here will offer you additional info."
