help w/69 Firebird carb sizing

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.  
Engine builder,self taught auto body guy.
Horsepower sells engines and torque wins races

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.  
Engine builder,self taught auto body guy.
Horsepower sells engines and torque wins races

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.  
Engine builder,self taught auto body guy.
Horsepower sells engines and torque wins races

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  

I've been having the same problem with my 72 pontiac grand prix 400 motor 30 over,bigger cam etc after overhauling the engine by a local machine shop engine was making a lower end knocking noise an the owner of the machine shop as me to remove the engine to take it apart,inspect it since the engine did not even have 50 miles on it i thought this was warranty but it turn out to be that i had to pay cause 6 of the eight pistons where crack or broken  and the bearings where score. he said that my carburator cause this i have a 750 holly dobble pumper which I bought new. the oil did have a fuel smell and the car was smokeing but it had plenty of power,I had try a quadra jet carb but it had no power thought it didn't smoke now the engine is fix but i need to buy another carb,I was thinking going with a different brand of carb. any ideas.  

Hello

If you have the money just buy a new smaller carb. I would stick with the square bore style holley you currently have. I know that no one else here would do that. They are all square bore “haters”, that’s ok? I’m put driving my car while they are still trying to change springs, needles, jets and apt screw steetngs etc. lol I have an enormous amount of holley experience a bit less with rochesters, q jets etc but I can still easily tune them. So if you want a HOLLEY this is what I would suggest.

A Q Jet will get better mileage than a comparable size holley.

A vacuum secondary is for low performance driving ie cars with small cams like a 256 etc.

There is no difference in mileage between a vacuum secondary and mechanical secondary if you drive them both conservatively.
A holley is much easier to tune by a mile for a novice no matter how much everyone tells you how easy it is to tune a q jet. If you don’t believe me just look at the recent post by buaainhalfdoz titled “q jet help needed”. lol
The easiest thing to do is sell your800 on ebay for $200 and buy a new 650 from summit for $400.

A. If you want to try to tune your 800. Try this then give up if it is still too too rich.

1. make sure your power valve is not dried up. It should depress easily.

2. it must be no higher number than a 6.5 power valve. 10 is high 1 is low. 6.5 is used for virtually all street applications.

3. front jets should be 70 max 65 min. if yours are larger than 70 that’s a big part of your problem.

4. rear jets should be 68 max to 63 min.

5. if you use 4 #68 all around with a 6.5 power valve it should be very close. If a lean flat spot [hesitation] occurs go up 2 sizes on the mains.

6. if a rich burble occurs than go down 2 on the mains.

7. if it won’t work sell it go to plan B.


B. Buy a 700 square bore 4150 double pumper ar equilavent holley vacuum secondary carb.

1. same jet recommendations for the 650. It should come jetted close as is.
2. if you use a vacuum secondary you absolutely buy the optional vacuum secondary spring kit. Install the white one. It will open the soonest and it will be happy.

Good luck  

Hello

Your 800 is not necessarily too big, it is but, if you floor bog any double pumper in a car with a small cam or tall gears at low rpm's it will load up the plugs.

If you have the money just buy a new smaller carb. I would stick with the square bore style holley you currently have. I know that no one else here would do that. They are all square bore “haters”, that’s ok? I’m out driving my car while they are still trying to change springs, needles, jets and apt screw steetngs etc. lol I have an enormous amount of holley experience a bit less with rochesters, q jets etc but I can still easily tune them. So if you want a HOLLEY this is what I would suggest.

A Q Jet will get better mileage than a comparable size holley.

A vacuum secondary is for low performance driving ie cars with small cams like a 256 etc.

There is no difference in mileage between a vacuum secondary and mechanical secondary if you drive them both conservatively.

A holley is much easier to tune by a mile for a novice no matter how much everyone tells you how easy it is to tune a q jet. If you don’t believe me just look at the recent post by buaainhalfdoz titled “q jet help needed”. lol
The easiest thing to do is sell your800 on ebay for $200 and buy a new 650 from summit for $400.

A. If you want to try to tune your 800. Try this then give up if it is still too rich.

1. make sure your power valve is not dried up if you didn't replace it. It should depress easily.

2. the power valve must be no higher number than a 6.5 power valve. 10 is high 1 is low. 6.5 is used for virtually all street applications. it is marked on it.

3. front jets should be 70 max 65 min. if yours are larger than 70 that’s a big part of your problem.

4. rear jets should be 68 max to 63 min.

5. if you use 4 #68 jets all around with a 6.5 power valve it should be very close. If a lean flat spot [hesitation] occurs go up 2 sizes on the mains.

6. if a rich burble occurs than go down 2 on the mains.

7. if it won’t work sell it go to plan B it's just too big or your foot is lol.

B. Buy a 700 square bore 4150 double pumper or equilavent holley vacuum secondary carb.

1. same jet recommendations for the 650. It should come jetted close as is.

2. if you use a vacuum secondary you absolutely buy the optional vacuum secondary spring kit. Install the white one. It will open the soonest and it will be happy.

Good luck  

fordmastertech:
I've been having the same problem with my 72 pontiac grand prix 400 motor 30 over,bigger cam etc after overhauling the engine by a local machine shop engine was making a lower end knocking noise an the owner of the machine shop as me to remove the engine to take it apart,inspect it since the engine did not even have 50 miles on it i thought this was warranty but it turn out to be that i had to pay cause 6 of the eight pistons where crack or broken  and the bearings where score. he said that my carburator cause this i have a 750 holly dobble pumper which I bought new. the oil did have a fuel smell and the car was smokeing but it had plenty of power,I had try a quadra jet carb but it had no power thought it didn't smoke now the engine is fix but i need to buy another carb,I was thinking going with a different brand of carb. any ideas.

Please start a new thread with your questions and not high jack someone else's.
You will get more replies that way.  
Engine builder,self taught auto body guy.
Horsepower sells engines and torque wins races