Well my question is this a stock marine carb has 88/94 power valves front and rear. This holley 850hp new out of box has 80/80 power valve only in the front.. How can they be co different ?

88/94 with front AND rear powervalves, is a heck of a lot more fuel than 80/80 with pv in front only.

Theres more to carbs than jet size. Things like booster design, venturi size, air bleeds, powe valve restriction diameter, idle jet size, and other factors that determine a carbs calibration.

Point is, comparing jet sizes between different carbs is useless. You can slap a 750 on with 75-81 jets, and get the same air fuel ratio as say a dominator with 92/98 jets for example

As stated. when a engine is under constant load is looses vacumn in the intake. Boats are loaded.. low intake vacumn as compared to same engine combo in a chevelle with a 3.73 gear at 55 mph.

Not trying to restate what has been nicely said above.

You need to compensation for lack of vacuum that a constant loaded boat engine experiences. Vacumn is what signals the pull over fuel from the metering circuit and venturies under cruise speed to keep fuel ratios a a good and safe level.

Carb air bleeds are more sensitive to enrichen and jetting is increased to give a marine carb a richer condition with less vacuum in the intake. (air fuel calibration ) . You don't want a hesitant. lugging.. pinging (if you can even tell with open exhaust) big block under load.

Stay with the marine application set up .

Anything with square jetting (same sec jets as primary) should have 2 power valves. So, I don't get the reasoning behind that 850HP.

As far as why carbs have different jetting but with similar cfm ratings:

1) Jets are not the only thing that influence fuel rates. We have different booster designs, different venturi and throttle plate sizing. Inside the metering blocks are emulsion tubes that can differ, channel sizes and restrictions, etc,etc,etc.etc list goes on

2) Different use applications ranging from: sing carb drag boat, dual carb drag boat, single carb smaller cid, single carb larger cid, different range of camshafts, etc,etc,etc,etc yeh, list goes on

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Good example,
There was/is a 830cfm Holley for Automobiles. I can't tell you how many people bought this thing for there street big blocks. Was absolutely horrible carb on these. Time and time again, the talk was 'it's too small' or 'it's too big' and etc,etc,etc as it thru fits since people couldn't seem to tune them correctly.

So....a person like me (after giving up) sits down with their Holley reference material, does some digging, and finds out it was desigjned for small block road course racing.

Bingo !

LOL. I guess coming back to finish my post after starting it a long time ago was a waste. Hah !

I see you answered what I did.

Easier if I waited and just replied 'X 2.' LOL.

The off road guys use a different vent that serves the same purpose as a J tube without the issues. Both vents are connected with one tube that has holes across the top with an offset to miss the air cleaner hold down bolt.

Best bet is to send your carb to a marine carb. performance shop and have it flowed and setup to your engine specs.They cut the choke horn off, seals on the throttle shafts, and alot more to the metering blocks as for jtubes they found that they caused a restriction so they switched back to straight tubes. They did a stage 5 on my carbs, 6 years ago.. never a problem, never a hesitation, backfire etc.
I used http://nickersonperformance.com/marine/marine.html Ask for Brad or Dean if interested. Located in Bristol PA

Are you talking about the QFT M850? If so, it doesn't have a PV in back, only the front. I have one and have had it apart.

It was this one. https://www.holley.com/products/fuel...arburetors/hp/

Ever put an o2 sensor on your engines with their carbs?