Racing Fuel Systems

So I have a issue I can't seem to solve. I have two 4150 carbs sideways mounted on a tunnel ram. They had standard .120 holley needle and seats. I switched to blp .120 bottom feed needle and seats trying to fix my problem. It has wedged nitro floats. I'm running 6 psi of fuel pressure that has been checked at multiple locations by multiple gauges. I'm also running old skool filter less velocity stacks. Everytime I run this engine in my driveway the float levels creep to the top of the rear sight glass. The carbs are at a 3.5 degree angle so I've been trying to set them halfway up the rear sight glass or just a little below. One thing I want to note is the fuel levels WILL NOT increase with the pump on and the motor NOT running. It only does it with the motor running. I have had the floats put and tested in a bowl of fuel and all 4 are good. Beginning to wonder if the velocity stacks are screwing with the bowl vents. Any input would be appreciated. Thanks

Are the gauges you are using for testing liquid filled?

I assume you are running gas, not alcohol. Is your engine capable of 2000+ hp? Because that is what those large n/seats support.

Size your n/seats for the expected power output [ probably much smaller than 0.120" ] & your fuel level will remain constant.

standard size is .110 since forever and always, Suggest you try them.

you've researched it wrong. .110 is the standard size.

area of .110 orifice = 0.00950334

area of .120 orifice = 0.01130976

your 120 orifice is about 119.008264463 larger than than the 110 orifice. 20% is not a small change.

the fact that the level creeps up while the engine is idling but not when it's off strongly suggests there's not enough float buoyancy to keep the needle seated. try a stock float it will do better.

Four needles & seats 0.120" @ 6psi. From the original Holley book ONE 0.120" n/s flows 295 lbs /hr, 4 flow 1180 lbs/hr.
Typically 0.5lb/hr per HP, so 2360 HP. That is theoretical, with no safety margin for delivery to the carb.

You use the smallest size n/s [ not the biggest ] that delivers enough fuel for the expected HP to maintain a steady fuel level. Since you have 2 carbs, you have 4 n/seats to supply fuel, no reason for large n/seats.

I will be installing the four .110's today and trying it. Wonder why the well known "carb builder" didn't follow this. These carbs where supposedly built specific for my application which was a sub 700hp tunnel ram application. Thanks for the help