In my never ending desire to squeeze the most from whatever I'm fooling with I come to you guys with this puzzling carburetor question. I have asked many people and even asked holley with no definite answer. I know I could pull the carb off and measure the bores but before making a mess on the intake I wondered if someone may be able to help me.
Carburetor has 4780-7 on the choke plate (should be 800 cfm 71 pri 85 sec) however it also has 6r 3767 b on the center section holley says that should be a 750cfm...????

I have this carburetor on a gen6 502 that is .030 over with 10:1 compression(509ci). I purchased the engine as a rebuilt unit that had not been fired yet. It came with an 850 on it and was very dingy and dirty so I opted to use my "800" that I had on my old engine(476ci). The engine feels like it has little to no difference in power at cruise (primary open, secondary closed) once I open the secondary it comes alive. I'm wondering if this isn't a 750cfm carb and it is holding back some hp. This engine should be around 600 hp while my old engine was 425 hp. Not seeing a difference in the cruise speed/rpm leaves me looking at the carb. Running a v8-24 module and base timing is at 9°.

Any suggestions would be greatly appreciated.

What Rpm are you aiming for? Summit has a fun little calculator for this.

https://www.summitracing.com/experta...cfm-calculator

If you're turning 5500 on a 509 it comes out to needing a 900cfm carb. After a few conversations here it seems that even my 850QF may be marginal on my 500hp 502. Since boats spend most of their time in the secondaries anyways oversizing a smidge isn't as big of a deal like it is on a car that spends its life at 20% throttle most of the time.

5200 Is a safe number in my mind. Im tempted to open the 850 up and rework it to see how it does. I guess I'm an odd ball boater, I try to stay in the primary just before the secondary opens. Kind of thinking about fuel economy if I could get it to cruise there at around 40mph I'd be happy. Heck I could cruise 38~+ - at 3800 rpm with the old engine on a 22 bravo 1. I was really expecting this one to just twist right on past that with the same amount of throttle given.

I'm still kind of stumped as to why my carb has 2 markings that don't match up.

The two stamps I can't answer for, but if you put a vacuum gauge on the manifold port and start seeing X amount of vacuum at wot then the carb is a restriction and you need a bigger one for wide open gains, if that's a goal.

Stamp# on center section is the center section's casting #.
Stamp# on the choke tower is the carburetor list #.

Since different cfm and list # carb's can share the same main body casting #, always go by the list #.

Just so you know, the 750 and 800's use the same throttle plate size. They also have the same primary venturi size. The 800 has slightly larger secondary venturi size.

for comparison sakes:

750cfm
P Venturi S Venturi P Throttle Bore S Throttle Bore
1-3/8 1-3/8 1-11/16 1-11/16

800cfm
P Venturi S Venturi P Throttle Bore S Throttle Bore
1-3/8 1-7/16 1-11/16 1-11/16

850cfm
P Venturi S Venturi P Throttle Bore S Throttle Bore
1-9/16 1-9/16 1-3/4 1-3/4

Thank you for the helpful information. I will try to check it with a vacuum gauge today and see what it's doing.

SB, thank you for all of the specific information on the two carbs.

Are you running the same prop you did with the old engine? If so, your cruise rpm shouldnt change at all, irregardless of the horsepower gain.

Hunter, Don't let it run lean.

Does anybody have a link to that whole holley list number list? I can only find a partial one. Thx.

This is where I got the info.
Holley carburetor specifications, list numbers, jetting and tuning tips.

Mr gary, I have checked the plugs it isn't leaning out.