that kn velocity stack can really mess with your float levels especially if you are using the marine j-tubes. These carbs are designed to get their airflow from the sides not from the top.

We made about twenty five pulls just changing air cleaners, stacks and sub stacks and two carbs . Best combination was the hp carb design with a fairly low profile air-cleaner that hugs the top of the carb.

The worst in any combination was the velocity stack and j-tube combination.

using a milled top 750 DP std vents, used the v-stack because it puts the filter into the scoop opening, going to try a flowed 850 I have laying around next year

Yes = swapped an RPM air gap for a 4150HP for a Brodix single plane for the same carb
No = changed cam to complement the increase in air flow

Why change. Dual plane and original cam squashed the air flow capabilities of the heads and exhaust. Not all the fault of the intake, but it has it's limitations.

If you have a STOCK ZZ502 motor, keep the stock manifold or a minor upgrade would be to use the Performer Air Gap. From my experience, for marine use, get rid of that ridiculous vacum secondary carb and purchase (JEGS or Summit) a Holly 4150HP 950 mechanical. Minor jet change to your motor needs. That and the intake change will net you some additional power without major motor work.

5500 rpm or less the biggest carb you want is a 800 cfm 5000 rpm stay with a 750 cfm jetted right any thing bigger cfm carb is just over carbing the engine in that rpm range the engine will never flow anything bigger.and you will have very slow sloppy air signal for the carb to funtion right. slow ventury means weak lag throttle responce = Sh!!ty performance.

Sorry to disappoint you, but RPM only plays a small part in what the motor needs for a carb. It's air flow. That dictated by the heads, cam, intake and exhaust and then RPM. I know, the cam in part dictates the RPM.
Second, check out what really makes up the 950 #4150HP carb. You will be surprised. Hint: Just because it flows 950cfm doesn't mean that it contains those exact parts.

Ok! you make no sense........think about it no rpm = no airflow your a friggen genius! rpm = airflow the faster it spins the more air flow you need per cycle and yes the cam is what makes up the time per cycle on the amount of air per revolution per cycle.if you need more information I would be gladly explain to you on airflow needs per cycle.

Carb Sizing formular is:
CARB CFM = Cubic Inches x Max RPM's / 3456 x VE%

VE is Volumetric Efficency. 100% is not likely
A good built motor is around 90%

502 x 5200RPM /3456 =755.32 CFM Carb
755.32 x 90% = 679.79 .

502 x 5600RPM /3456 =813.43 CFM Carb
813.43 x 90% = 732.09


The HP 500 used a 800CFM

If I'm not mistaken the 950 holley is just a race version of a 750 not actually a 950 cfm carb...

Hey Einstein, put your reading glasses on. I never wrote "no rpm". I wrote "only plays a small part".
Your VE graph is just that, a graph. Uses only the Cubic displacement and the rpm. Hey Einstein, according to your graph, the heads, cam valve size, intake manifold don't matter. Hell, just use your graph and slap on what cfm carb size your graph spits out. No I don't need to explain air flow. Take some useful information from some professional head tuners. They are NOT using the flow bench information as in the past. It's TEST, TEST and more TESTING.
I have a name for your graph theory slap on bolt it up motor build-up: Cookie Cutter Engineering