I have twin 502's on my 33 powerplay. I'm getting a lift installed at the house so it will be staying on a lift. I want to add a fresh water flushing system to each engine as it will be hard to put the flushing ears I have now on the out drives when it's on the lift. My question is I see the systems have a valve that you can close and open, does that mean you have to turn that valve off so you can flush from your hose connect? The valve seems like it would go by the sea water pump and it seems it would be a PIA to have to turn that valve off and on for each engine when it's time to flush. Thanks for any help.

The first question is, do you have sea strainers in your boat? If so, do they have a spare port that you can attach a fitting to? If the answer to both of these questions is yes, then you can install a Perko hose inlet, plumb that to a tee and the plumb the tee to the two strainers and now you can direct water right to the strainers. In this scenario, I would still recommend having valves between the tee and the individual strainers as you should only flush one engine at a time on hose pressure and you will not want it to bleed off going to the other non-running strainer. If you do not have strainers, there are other options but I'd need to know more about your setup to recommend something.

As for the valves you are referring to, where specifically are they in the flow loop? IE: are they controlling freshwater to the pump or are they shut off valves that cut off the raw water from the drives ahead of the flush system inlets? if they are the latter, then they are there so you can still flush the engines with the boat in the water.

If you can be more specific about what you have in the cooling loop and even post pictures, it would be easier to steer you in a direction. I've done a few of these flush systems already.

My fresh water flush just T's into the incoming water line coming from the transom to the sea strainer.

I actually also put a large stainless steel valve just before that T so that if I spend the night somewhere I can close off the sea water and flush the motors at a marina. The people at the marinas don't always love that but at least I know salt water never stays in my motors.....

The ones that have a valve installed are typically used to cut off sea-water, so you can flush the motor while the boat is still sitting in salt-water.
It's of obvious mondo-importance to re-open those valves before you attempt to go boating again.

You do not need valves, as long as your fresh-water hose insertion is directly in front of the sea-water pump, or directly into the sea-strainer as mentioned.
As a matter of fact, when you DON'T have a valve, and turn on the fresh water hose, it will flush backwards out the drive. When you start the motor, it will suck the water into the SWP and flush the motor.

Thanks to everyone for their replies. I don't have sea strainers. I just have the sea water pump. If I'm understanding correctly you never have to touch the valve that comes with the flushing system unless your boat is in the water and you want to flush the motors?

Do you make a T on the hose coming from the transom assembly before the sea water pump or after? Just trying to figure out if it matters.

Must be between the transom assembly and the sea-water pump; as close to the SWP as possible.

FWIW. Also be sure that when you are not flushing and running with seawater coming in from the drives that the caps at the flush fittings are properly tightened and have good hose gaskets or you may suck some air and reduced cooling water being sucked in.