Let's also think about an over-carb'd situation...... if the carb flows too much are, the the velocity of the air thru the venturi is slower and will not atomize the fuel as well.... that is provided the engine not capable of handling the extra air. The result would be a "bog" when accelerating most probably. As stated by Cord earlier, a 100% efficient 502 would require a 755 cfm carb.....you are not even approching 90% efficient so why go bigger??
A carb larger than required by the engine will actually make it perform worse.... just like someone putting a 750cfm on a 305 gm or Jeep engine.....it will suffer badly...been there done that!!(many years ago).

I have a Holley 750 with Vacume secondarys for sale if your interested. $150 and its the marine version. It was the stock carb on an OMC 330 hp 454 King cobra. I went ahead and rebuilt with all new parts from the rebuild kit. I switched to a mechanical secondary. I like it better but thats not helping me sell you this carb. I have the spring kit and any power valves (this takes two of them) or jets you would want.

And now back to our regularly scheduled program

I see alot of talk here about different carbs. But nobody has mentioned anything about the rest of the fuel system. What kind of pump, size line, etc.. do you have? A larger carb will only run leaner if you can't feed it enough fuel. You have to feed the monster first. It sounds to me like you may have had a supply problem that contributed to this condition.

blown formula, I agree. Too big is not good either. The stock 502 carb was an 800cfm. I looked for a used stock carb from a GenV pre-fuel injection but couldn't find one so I used the 750 that I had.

In the Edelbrock Catalog there is a section in the back called Technical Discussion. It has all the rules for figuring Volumetric Efficiency and CFM and Carbs. General information only but, according to the charts a 502 at 100% efficiency requires 871 cfm at 5000 rpm. I doubt if a stocker is 100% efficient so an 800 or even an 850 should be in the ball park. Since I do a lot more cruising than WOT an 800 sounds about right for me.


Intolerant1, Thanks for the offer. I'll think about it but, I think I want to go with mechanical secondarys, too.

Checkmate454mag, Good question. I am using the stock fuel pump and lines. When I put on a new carb I will also put a fuel pressure guage on so I can check it.


Thanks everyone for your replys. Keep them coming.

Mac
I work on carburetor design for a living. Carburetors work based on air velocity. A carburetor that is too small on an engine (provided that it is in proper working order) will run richer than a larger carburetor with the same size jets in it.

Basic carburetor 101. An engine is an air pump, once it is built in will only flow a certain amount of air (unless you overly restrict it) Now based on Bernuli principle and the venturi affect as a liquid gets pulled through any hole it creates low pressure, and high velocity. Now back to our engine your crank spins over, the intake valve is open, and the piston moves downward creating a void. Now atmosheric pressure (approx. 14.7 psi @ sea level) forces air into the engine to fill this void. As the air travels through the venturi of the carburetor it creates a low pressure area. You have fuel in your float bowls pressurized at atmosheric, and low pressure in the carburetor the fuel is now siphoned into the engine. So put a smaller carburetor more vacuum sucks more fuel, bigger carburetor less vacuum sucks less fuel. This is over simplified.

Cord & anyone else trying to use the old CFM formula.
IT DOESN'T work. The reason being is it doesn't take your engine into consideration. Take two 502 engines both going 5000 RPM change the compression ratio from 8:1 to 14:1 not the same requirements. The next problem is that there is no standard for testing carburetors to compare them. A 750 Holley, will not flow the same as a 750 Edelbrock, or 750 Barry Grant. So you have to go based on what that manufacture recommends in their own product line. On this combination a 750 would be the proper choice.

Doug

RumRunner, I am not sure I understand what you are saying. Are you saying that a carb would always run richer if it is put on a bigger engine, or only if it was too small for the bigger engine? When I first tried the carb with stock jetting from the 454 on the 502, it was defineitly too lean. It would backfire whenever the throttle was pushed hard past midway and then take off. Bigger jets cured that problem. I ended up 3 sizes bigger on the primarys and 6 sizes bigger on the secondarys. On an Edelbrock there are needles that run in the primarys so that takes away some flow. Stock, they come with primarys that are 2 to 3 sizes larger than the secondarys. I ended up with the secondarys only one size smaller than the primarys. If I have the correct size carb on there maybe I just need to richen it some more. What do you think?

Mac

Sorry if I wasn't clear. If your carb is working properly, and you put a larger carburetor (same type) on the engine it would run leaner ! I think there are better 750 carbs out there for your engine, but you didn't burn your piston because the carb was too small. It may have had a problem with it, if it even had anything to do with that at all.


Doug

Rum,

I am going to a Holley for sure. Just have to decide which one. Thanks.


Another Doug,

Most lean conditions will be seen at the outer cylinders. They are more likely to receive less fuel.(longer runners farthest from carb) Most plug readings are taken at #7 plug.

Tommy at Chief Engines had me setup with an 830 on a B&M 250 supercharger. So an 830 or less would be plenty for a NA motor.

Good Luck !

Mac-don't use the 4160 series. Stick with the 4150's. You want the metering plate, not the blocks. You won't need to run a choke, so it's optional. You don't need any of the race bodies.