# Compatibility Questions: 600 CFM Double Pumper with Pontiac 400



## 69-GTO (Dec 20, 2020)

Hi there,

To start, I should warn you all that I'm a novice to this whole automotive language and lifestyle. I have very limited time as a medical student but I'm trying this new hobby to restore my Dad's GTO after he passed away. I would appreciate any help and opinions regarding my goat's situation.

Engine in car was sitting in garage hasn't been started in years -- no general maintenance since about a decade ago. Threw a new battery in and started her up. The old carburetor (Holley 650 CFM double pumper) was severely leaking and threads were stripped all over from accelerator pump housings to fuel bowls. I decided to just get a new carb after the rebuild kit and tapping some new threads proved unsuccessful. Under _probably _not enough time for researching this, I ended up buying a brand new Holley Double Pumper but this one is 600 CFM. 

Link for reference: Holley 0-4776S 600 CFM Double Pumper Carburetor
Part # 0-4776S 

Looking at the second picture in this link, they show a table with Cubic Inches on y-axis and Max Engine RPM on x-axis. From what my father told me, the engine is a Pontiac 400._ (I struggled looking for codes. I found one on the front passenger side: 340469 and I think the engine code is YX. I found another on the back passenger side: 481988 which might be the block casting number.)_ As for researching max RPMs I pulled my best guestimate out of various forums and google searches to be somewhat under 6000 for my engine/condition. Is this accurate??

More importantly, taking all this information together, will this new carb work well for street performance? Holley's chart recommends this model for 400 cubic inches and 5500 max RPM. I hope I didn't mess up big time and get the wrong carb. Maybe I should take a break studying for 5 min and suck it up to call their tech service for once. 

NOTE: I have the carb installed and it operates like a champ no leaks but fuel pressure looks a bit strange regularly fluctuating 3-6psi (maybe just problem with new gauge?) and full manifold vacuum is regularly fluctuating in the low range (around 10-15). Timing seems way off and sounds terrible. Then again, it always kind of sounded off a few cylinders so I don't know how much this is a carburetion issue in addition to others...

ANY FEEDBACK HELPS AND IS MUCH APPRECIATED!
Thank you.


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## BearGFR (Aug 25, 2008)

If you want to slow the car down, that's a good choice  

In case you weren't aware, even the base level Rochester Quadrajet carbs that would have been "factory" on that 400 were capable of 750 cfm.

Despite what you might read on the internet, it's pretty hard to get a carb that's truly "too big" for a given engine. You'd have to go really HUGE on that 400 before that might start to be an issue - and by huge I mean a big 1100+ cfm Dominator. There's a show available on MotorTrend TV called "Engine Masters" where one of the things they do is run real world dyno tests on engines, often to to either prove or disprove various internet myths. One of the ones they did was on this very thing - is it possible to get a carb that's too big? Testing a 383 chevy, they started with a 650 (which was what all the online calculators said was the 'right' carb) and found that it was restricting flow. They went up to a 750 and it picked up both torque and power. The youtube edited version of the episode doesn't show it, but they also tried a crazy big Dominator on the engine and found that it didn't HURT the engine any, but it also didn't make any more power because the 750 was 'big enough'.

But, sounds like you already have a square bore manifold on the car and are committed to the "brand H" platform. I'm guessing your fuel pressure is fluctuating because vacuum (and probably engine RPM) are also fluctuating/surging?

If that's the case my first suspicion would be that the idle mixture is too lean. The best way to dial that in is to use the vacuum guage it looks like you already have. Turn both the screws in gently until you feel them bottom out, then back them out exactly two turns as a starting point. Start the car, let it warm up, then adjust them in equal increments until you find the setting that gives you the highest manifold vacuum reading. You might have to keep adjusting the idle RPM as you do that also.

See if that doesn't help things a little.

Bear


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## michaelfind (Dec 14, 2018)

Bear always gives solid advice. Pay attention when he replies. He's like the surgery attending that speaks truth through all the noise.

Which school and year are you in now? Doing some hands on tinkering may be a great stress reliever during the pre-clinical and clinical years. Just remember to keep your focus and know that running/tuning these fine machines is a gradual process. Let it be a fun learning process. Don't let it become a deadline and a frustration. I'm happy to know you are honoring your Dad in a way by fixing up his old Goat. My story is similar. That is a common theme on these forums, except now I'm starting to see more people fixing up Grandpa's GTO! Time goes on.


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## BearGFR (Aug 25, 2008)

michaelfind said:


> Bear always gives solid advice. Pay attention when he replies. He's like the surgery attending that speaks truth through all the noise.


Aww gee.... thanks for the kind words, sir


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## dan woodland (Jul 24, 2013)

michaelfind said:


> Bear always gives solid advice. Pay attention when he replies. He's like the surgery attending that speaks truth through all the noise.
> …


I will whole heartedly second that!!!


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## Boomstick (Sep 13, 2021)

so by reading the comments, i may have screwed up when i ditched the 750 holley my 69 came with and went with the 600 summit carb?


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## pontrc (Mar 18, 2020)

Boomstick said:


> so by reading the comments, i may have screwed up when i ditched the 750 holley my 69 came with and went with the 600 summit carb?


A lot of us are Q-Jet fans , good fuel economy and big Cfm,


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## PontiacJim (Dec 29, 2012)

Boomstick said:


> so by reading the comments, i may have screwed up when i ditched the 750 holley my 69 came with and went with the 600 summit carb?


Not necessarily. What you have done is limit the full potential of the engine as the engine may/may not get all the air flow (CFM's) it can use.

The 600 CFM carb may make your engine/car feel more responsive as air flow/port velocity may have increased. The problem may be that the 600 CFM carb will drop off in power earlier in the RPM range than the 750 Holley. For example, maybe the engine could be wound out to 5,600-5,800 RPM's with a good seat of the pants feel. Now with the 600 CFM, the engine may only want to pull hard up to 5,200 - 5,400 RPM's and then you will feel the HP go flat - ie the engine will continue to rev, but there is no power gain.

So it can boil down to what you want out of the engine/car and how you drive it. If you shift, or keep engine RPM's down, then not an issue. If you have a larger than stock cam, let's say no bigger than the factory "067" cam and keep RPM's down, not an issue. If you like to wind out the engine or have a larger cam, then you are simply not utilizing the potential of the engine/cam set-up.

Keep in mind the early GTO's with the single 4-Bbl AFB's were rated at 550 CFM's and used the "067" cam. The Tri-powers a little more with the '66 large outboard carbs flowing around 780 - 800 CFM's. The '65 uses th smaller end carbs which are said to flow 50 CFM's less, each, so 100CFM's total or 680 -700 CFM's and used the "068" cam.


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## lust4speed (Jul 5, 2019)

Fully agree with Jim. I played around with a Carter 600 AFB carb in place of a later Quadrajet. Idle and low speed response was better with the small carb, but about 4,600 RPM it slowly lost out to the larger unit and was definitely dropping off by 5,000 RPM. Engine is nothing more than a large air pump, and only needs more air at higher RPM. Up until then the engine doesn't need the extra carburetor capacity.

At our GTO club dyno day, my friend with a 400 engine sporting a mild roller cam wanted to try out my vacuum secondary 850 Holley in place of his 650 Edelbrock AFB. After a base run with the small carb we bolted on the larger. The 850 didn't loose any power down low, but only made eight horsepower more at the 5,200 RPM peak. Although he did have headers, the remainder of his 2-1/2" exhaust and stock cast iron heads and iron intake just didn't need any more carb, and it wasn't the limiting factor with his combination.

So if you are out cruising around and drop into passing gear and it upshifts on its own around 4,600 RPM you will not be noticeably slower with your 600 carb or quicker with a larger carb.

So, would I buy a 600 carb? No. Would I run it if I had it? Yes.


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## Boomstick (Sep 13, 2021)

I've got the 750 rebuilt. Beforehand, it bogged terrible off idle and letting off from wot it stunk like raw fuel, that's why I the the 600 summit on it and it's ran good ever since. The accelerator pump diaphragm and vacuum secondary diaphragm on the 750 were both hard as a rock so I went ahead and rebuilt the carb. I still haven't had time to get it on the car yet but I did take the car for a drive yesterday. Might work on it today...


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