I've got a 69 Firebird w/a mildly modified 400 Pontiac engine.  What I know:  It's a 69 model engine w/62 RA III casting heads, Edelbrock Performer RPM intake, 10.5:1 pistons, Hooker comp headers 1.75".  What I don't know: no idea what cam this thing has....It's not extreme.  I would guess around 270-282ish hydro.  The prob is the carb is a Holley 4150 800 double pumper and it's still giving me all kinds of trouble even after a fresh rebuild.  It's putting way too much fuel through the motor as evidenced in the condition of the plugs.  I've researched the origional specs and understand the factory equiped these cars with 750 Q jet's.  My main question is this.  Is my problem the CFM or the type of carb?  Mechanical secondary vs. vacuum secondary.  I want a solid street car not a racer.  

If you are correct on the cam size then that is way to much carb. for your engine I would recomend 600-650 cfm at the most.  

Probably not CFM. Thats the max air/fuel mixture flow of the carburetor, doesn't really count when you're just riding around.
The 800 would do better on a hotter cammed engine than yours, but with proper tuning it could be made to work. Thats not a kit and turning screws but a professional builder that would set it up to your engine specs. It requires changing different air bleeds and restrictors to get the carb working right.

For your application, something street friendly but still have some umph, should you decide to play some, a 650 double pumper would fit the bill. Accelerarion would be crisp and smooth.This is based on your intake being for a square bore carburetor.

Second option, if it'll fit the intake, would be a late model quadrajet. Late 70's quadrajets for Pontiacs are mostly if not all rated at 800 CFM. Difference is, they only flow what the engine pulls through them, so 800 would probably never be seen.
77 up quadrajets are very adjustable. Any driveabillity problems can be adjusted out, if needed. A little knowledge about them does help, but a simple rebuild with kit instructions is fairly easy.

Something else, give the ignition system a good look over, if the fires weak, you may not have carburetor trouble at all.

Vacuum secondary vs. mechanical is an on going great debate. Really comes down to driver preference. With the mechanical you have control of the four barrel, with vacuum it comes in when needed, that's wheather you want it to or not. Not a plus if you're shooting for gas milage.

Have someone else check your 800 out and overall state of tune; float levels, fuel mixture, timming, spark, vacuum advance operation, etc. You may not be in as bad a shape as you think.
If you have to have a carburetor, Holley 650 double pumper or well preped quadrajet, either one will make driving your Pontiac more fun.  

Thanks for the reply.  What confused me most was when I found out that the factory equiped these cars w/a 750cfm carb.  I thought that was way too much for the street.  I was thinking about doing the simplest thing I could by selling my Holley and buying a new Edelbrock matched to the intake I've already got.  I was hoping that would be more "plug and play".  What do ya'll think?  

If it was mine I would put a 600 CFM holley on it. also the 750 CFM rating on the Q-jet is probably equal to a 600CFM after market carb. by time they jet it down.  

Cool.  Is the Holley a plug n play lkike the Ebrock?  

There just like every other carb. they will need to be ajusted for your application for best results.  

Thanks for your help.  This is my first attempt at this and every little bit helps.  Is there some good reference material out there that could help?  

Hi av8tor33,

Back in the day, the 400 Pontiacs came form the factory with a 600 quadrajet carb that was great for street/strip applications.  The smaller primaries were good for cruising & "economy", and the massive secondaries supplied the umpf for wot sessions.  At some point, we must all acknowledge that the Pontiac engineers knew what they were doing when it came to designing and tuning these engines.  Now your 400 has been stepped on just a bit and it can now breath better than stock.  The 800 Q-jet or the vacuum secondary 750 Holley would be good choices, but they they will both be a little fat on the fuel side, and so will have to be tuned.  Please don't expect to take a new carb from the box and not have to tune it.  And, I mean tune it beyond turning the adjustment screws.

Keep in mind that any more fuel than is necessary to get the car down the road smoothly and with a happy, cool-running engine, is all wasted.  Also remember that the car is going to be cruising on the idle  circuit all the way through 2100 rpm or higher, so it is important to tune the idle circuit properly.  One of the tuning aims is to delay the onset of the primary circuit, and this is done by adjusting the the air/fuel ratio.  The adjusting screws, jets, metering rods and air bleeds will all come into play.

Follow this thread to a good article on reading plugs: HTTP://www.angelfire.com/f14/pontiacdude428/readplugs.html