I noticed that most if not all Merc carbureted engines come with a single power valve. I had Patrick at Pro-Systems build up a 950 for me and it has dual power valves. My concern is that I may be running to lean at cruise as I don't have the ability to check it with an O2 sensor. I will say that at cruise (50 - 60mph) I get 3mpg, so I'm not complaining just don't want to hurt the motor.

525SC came with dual power valves, if patrick did it for your app. run it !!!! I run one of his out of the box on my procharged 500hp great !!!

When the secondaries open, the secondary powervalve is going be open almost instantly anyway.

You're getting 3mpg @ 50 MPH what is your engine set up. Either way- thats great for a 28 foot boat

Nothing wrong with dual powervalves, if its all tuned properly.

I had a buddy with a pro systems carb built for his application. (procharged, 1200hp). If he ran it out of the box he'd have a pile of scrap right now. For starters, the single needle and seat bowls weren't supplying enough fuel, sucked bowls dry at wot. Went to dual needle and seat bowls and all good. Was not the pump ,it did it in the boat and the dyno. Jetting was way to lean up top for a marine engine to survive, and powervalve setup opened way too soon and dumped so much fuel when trying to plane it looked like a diesel out the tails.

Not trying to bash anyone, but I personally dont care if JC himself built the carb, chances are on a custom build, it will need to be tweaked. I'd rather double check myself because if the motor burns a piston or tulips valves, Pro systems aint gonna flip the repair bill.

I had PS 1000s (4150s) and PS 1200 (4500s) Both came with front and rear PV's and both ended up with PV's in the front only.
There was a lean spot at 3800-4200 prior to this.
You need an O2 sensor
All applications will be different...
Dave

Any suggestions on where to put an O2 on a set of Lightning headers?