Lol - did exactly that wth a Biggs carb on my 540/600 build years ago. Did not have to touch the Dom on the dyno - cause it was perfect. Lucky? Maybe... Maybe not, lol.

The power valve channel restriction in the carb sets the amount of fuel thru the circuit....not the Power Valve, If the restriction flows more than the PV, the PV becomes the restriction and that is not what you want, so you use a high flow power valve.

If you question hyat size your circuit is, get the high flow power valve.

Read that 10 times fast. :)

Edit in: yes, power valves from different manufacturers are interchangeable.

Ah yes, thank you. :)

i actually had had the best luck using the 4.5 high flow PVs and pulling back the jets some giving me lower 12s straight across the board with a tad richer at wot. I just switched the primaries from #78 to #76 to see if I can lean the pig out a bit.

Those tunes are what it takes to avoid lean conditions on accels and under load. If you want to build an engine to last you'd be wise to pay attention to what they're doing.

A problem for the aftermarket tuning world is that they interpret an engine making more power as it "wanting" to run at a lean condition. Engine masters had to try to regulate this a number of years ago as all the engines they tested were pinging like crazy.

I'd be leery of targeting 13 especially in a boat. Even if you have EFI the cylinder to cylinder air flow is variable and you'll definitely end up with a lean hole. Just look at some head flow tests where they measure all of the ports.

laugh if you want, but merc does real endurance validation. Cycling between peak torque and peak power for some number of hours.

Did you monitor air fuel ratio's once in the boat at various rpms/loads?

Boat "cruise" and car "cruise", are entirely different. Not remotely comparable. My car the other day, showed it was using 24HP to cruise at 70mph. Not a whole lot of heat being made. Now take a boat. Some boats may need 500, 600 horsepower to maintain 70mph. Hence why one engine is using a gallon of gas to go 25 miles, and the other is using a gallon of gas to go 1 mile at the same speed.

I dropped the primaries from 78 to 76 and now it's 12.7-12.9 across the range with wot about 12.8. Does that seem acceptable to the OSO brain trust?

Sounds lean to me. Especially considering that scientifically, textbook gasoline engines make max power at 12.5 :1 air fuel ratio.

would i run my car at wot for a quarter mile with afr of 12.7,maybe.will i run a n/a boat wot for more than 20 seconds with that afr,not a chance.