# New carburetor GTO 65



## adamrysz87 (Nov 14, 2021)

Hello, in my GTO someone had changed the carburetor before I bought it. 
This is Quadrajet 7042218 for Chevrolet truck?









I have the engine block code S1 = '61 389CI maybe more cubic... head C048 = '68 and intake manifold ederblock Performance Pontiac. 
Which carburetor do you recommend for this set?
I found a red and black electric choke cable, I think?


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## AZTempest (Jun 11, 2019)

From a quick search on your carb, yes, it does seem to be of a truck. I'm finding a Chevy 402 so I would think it could be a good match performance wise. The truck carbs I have been reading about to avoid were post 1975 qjets on some trucks as they had a .036 diameter power tip on their metering rods as opposed to the .026 power tip. 

The only thing I notice, and that's only because I was running a Chevy qjet on my Tempest, was that the fuel inlet is pointing to the side where Pontiac/Olds had them in the front of the carb. I wanted to be a bit more true to original and recently found an qjet of a Olds I have been playing with.


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## theamcguy (Jan 14, 2005)

Stock 1965 GTOs came with a Carter AFB number 3895 Manual Trans 3896 Auto Trans


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## adamrysz87 (Nov 14, 2021)

Thanks for the recommendation, but this is my first American car and I see a carburetor first time.
For now, I prefer to put on something that will match what I already have - cables electric choke, I think so...

What's the difference between the Ederblock 500CFM, 600CFM, 650CFM, 750CFM, 800CFM, and AVS2 and Performer Series ?


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## PontiacJim (Dec 29, 2012)

adamrysz87 said:


> Thanks for the recommendation, but this is my first American car and I see a carburetor first time.
> For now, I prefer to put on something that will match what I already have - cables electric choke, I think so...
> 
> What's the difference between the Ederblock 500CFM, 600CFM, 650CFM, 750CFM, 800CFM, and AVS2 and Performer Series ?



Your carb seems to have an adapter under it. So my guess is that the Edlebrock intake has the "square bore" carb pattern versus the "spread bore" carb pattern of the Q-jet.

The Q-jet was the factory carb from 1967 and up - has small primary bore openings and big secondary bore openings. Made by Rochester and Carter. Edelbrock continued the carb type after they were no longer made for production automobiles. Good on gas because the primaries were small, but plenty of air/fuel once the big secondaries were opened up. Has a unique sound when kicked open. Q-jet was used on most GM made cars of the era - so readily available. Differences do exist as they were modified over the years and the gas inlet port is different depending on the car make. The Chevy has the gas inlet on the passenger side. The fuel pump on a Pontiac is on the driver's side, so the correct fuel inlet is directly in the front of the carb and a steel line connects it to the fuel pump.

The gas pedal throttle linkage may be different between Chevy & Pontiac, so that can cause a problem. You can most likely adapt the throttle arm on the Chevy carb to work/function with the Pontiac gas pedal cable - should not be too difficult, just make sure the carb will fully open and shut using the gas pedal.

You can use the Chevy Q-jet, but I would make a steel line with a "U" in it to bring the line around to the driver's side and down to the fuel pump. 

The Edlebrock AFB (Aluminum Four Barrel) series you listed are the CFM (Cubic Feet per Minute) ratings, or size, of the carburetor flow. If you work with the Weber type carbs, I believe these come in different sizes based on the engine size or HP. Same thing with the Edelbrock carbs, they come in different sizes designed for an engine based on cubic inches or horsepower levels. The factory Q-jet flows 750 CFM's with some later carbs flowing 800 CFM's.

The AFB was introduced in 1957 on American cars and made by Carter. Edelbrock is an aftermarket car company that took over the AFB style of carburetor and has the rights to make them.

The AVS is an improvement over the AFB. AVS means Air Valve Secondaries. The secondaries on an AFB as an air valve that is weighted and when enough airflow is passing through the secondaries, it will pull the air valve open. To dial in this type of air valve, you could file the weights down to make them lighter and the valve would open faster, or add weight to make the valve open later. This was used to eliminate any engine stumble when the secondaries were opened up and the engine sucked in a big gulp of air just before the air velocity created in the secondaries pulled the needed mixture of air/fuel.

The AVS has a spring loaded/operated secondary air valve just like the Q-jet has. It can be easily adjusted by tightening a spring that will add more tension or take away tension on the spring so the air valve can be made to open sooner or open later - no weights to adjust like the AFB. It was often found on the high-performance Chrysler brand of cars.

The AVS-2 is another Edlebrock improvement on the AVS. Edlebrock designed/added annular booster rings which will atomize the gas much better than the AVS. So it is an improved design for better air/fuel mixtures, plus has the spring loaded secondary air valve.

You can use the Chevy carb and adapter, but adapters can disrupt the air flow because you are reducing the spread bore base of the Q-jet down onto a square bore intake. You would be better to use a square bore carb on a square bore intake and a spread bore carb on a spread bore intake.

Since the factory Q-jet is rated at 750 CFM and has "vacuum secondaries," you want a carb to match that if you do not use the Q-jet. 

Pic #1 is the Q-jet Spread Bore - small primaries & large secondaries

Pic #2 is the AFB Square Bore - primaries & secondaries are about the same

Pic #3 is the Square Bore manifold pattern on left, Spread Bore manifold pattern on right


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## adamrysz87 (Nov 14, 2021)

If I understand correctly I need 750 CFM.

This is my manifold, looks more like a Square Bore or Spread Bore ?









This is adapter and Q-jet.

















Most people buying on ebay "Holley Quickfuel Brawler 750 CFM Double Pumper 4 Barrel Carb E-Choke" is a good choice? If I want to replace the carburettor, I need to buy a few things. I'm sure the air filter ... what else?

What interesting i found hidden behind the engine - abandoned throttle cable bracket. Should I also replace it when changing the carburetor?








Other photo of connected wires









Fuel filter under the car.


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## AZTempest (Jun 11, 2019)

Your intake is a Spread Bore. Why they had an adapter for the Qjet you have on there I'm not sure. The Qjet should go right on without the adapter as it is a spread bore carb. May I ask why you feel the need to change the carb?

Interesting that the fuel line runs aft. Are you running an electric fuel pump?


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## adamrysz87 (Nov 14, 2021)

The main reason for replacing the carburetor is the prepared electrical installation for the electric choke. Manual choke link is missing, so I don't want to change it now. It will be easier for me.
I can't see the fuel pump. Maybe it's in the tank.

What carb do you recommend with eletric choke for this intake?


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## PontiacJim (Dec 29, 2012)

adamrysz87 said:


> The main reason for replacing the carburetor is the prepared electrical installation for the electric choke. Manual choke link is missing, so I don't want to change it now. It will be easier for me.
> I can't see the fuel pump. Maybe it's in the tank.
> 
> What carb do you recommend with eletric choke for this intake?


Why not buy an electric choke kit and add it to the carb you have along with a new linkage rod? May save some money as long as the carb is good. Contact Mike's with your carb number and see what kit he may have and if he can supply the missing linkage.









Divorced Choke Electric Choke Conversion Kit







www.carburetor-parts.com





You can also go with a new rebuilt Q-jet if you want to go that route. Several companies off these.

Looks like you have a *Lokar* brand throttle cable and bracket. That is not factory. You may have to do some adapting or adjusting to get it correct.

If you have an electric fuel pump, you need to know what the fuel pressure is and you may need a fuel pressure regulator up at the carb. You do not want more than 5-6 pounds of fuel pressure or you will flood the carb and you may experience a fire that burns down your car.

Get rid of all that rubber fuel line and replace it with 3/8" steel line. Only use the shortest amount of rubber fuel line. Make sure the rubber fuel line is ethanol gas compatible or the rubber will deteriorate and swell and split and you will have another gas fire on your hands.


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## adamrysz87 (Nov 14, 2021)

OK, thanks Jim. Where can I find a fuel pump?


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## adamrysz87 (Nov 14, 2021)

Has anyone ordered a carburetor from SMI? They can build and adjust the carburetor to the specification of the car. They offered to me Rochester 4BBL Quadrajet 800CFM.






Rochester 4BBL Quadrajet 800CFM for Pontiac


SMI Carburetor offers the best performing Rochester 4BBL Quadrajet 800CFM for Pontiac carburetor




www.smicarburetor.com




.

They will tune carb to my GTO if I give them information.
Is there a way to get the specs of an engine without disassembling ?

Jim helped me decrypt my unusual engine.
Block code S1 is 1961, 389 CI, 267 HP / 10.25 compression maybe after tuning to 455 CI .... Heads and transmission from 1968.

The most difficult questions are:
Velve Size, 
Comp. Ratio
Style Hydraulic, Roller or Solid
Duration @ .50 Int. Exh., 
Velve Lift Int. Exh., 
Label Separation Angel, 
Rocker Arm Ratio, 
which means ported ... 
and marked in red area.

Thanks for help.


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## PontiacJim (Dec 29, 2012)

adamrysz87 said:


> OK, thanks Jim. Where can I find a fuel pump?



Anywhere. Should be a standard fuel pump. There are different designs, but all bolt up AS LONG AS whomever rebuilt the engine installed the cam gear eccentric that the fuel pump arm rides on. If it was not installed for some reason, the the fuel pump will not operate.

The fuel pump on the bottom with have either 2 or 3 fittings. The 2 fitting pumps are for fuel in and fuel out to the carb. The 3 fitting pumps use fuel in , fuel out, and a 1/4" fitting for a fuel return line back to the gas tank - and I believe were introduced in 1968. Your '65 most likely does not have the 1/4" return line unless the car was originally an AC equipped car or if someone added it. The 1/4" line is to help with vapor lock as it circulates some of the gas back to the tank. The earlier cars would have used the 2 fitting pump and then a fuel filter near the carb that had a thrid nipple to direct the gas down along the engine and back to the tank.

If you get a 3 fitting pump, you can cap off the un-needed 1/4" fuel line.

Try RockAuto. They can be helpful on some parts and have good prices. I have purchased parts from them with no disappointments.

The Carter pump should work.






1961 PONTIAC BONNEVILLE 6.4L 389cid V8 Fuel Pump | RockAuto


RockAuto ships auto parts and body parts from over 300 manufacturers to customers' doors worldwide, all at warehouse prices. Easy to use parts catalog.



www.rockauto.com


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## PontiacJim (Dec 29, 2012)

adamrysz87 said:


> Has anyone ordered a carburetor from SMI? They can build and adjust the carburetor to the specification of the car. They offered to me Rochester 4BBL Quadrajet 800CFM.
> 
> 
> 
> ...


Not something you can guess at. You need to know what your cam specs are from the manufacturer. If you don't know what cam was installed, then no way of determining the numbers needed for the specification sheet.

1968 Heads #16, used 2.11" diameter intake, 1.77" exhaust.

Compression - 10.25

Stock rocker arms are 1.5 ratio.

You most likely have stock heads, no "porting" work.

I would guess you have a hydraulic lifter camshaft, not a roller cam or solid lifters.

I would use the specs for a 1968 Pontiac "067" cam which is:

273 degrees intake/289 degrees exhaust advertised duration
197degrees intake/212 degrees exhaust duration @ .050"
Valve Lift - .407" Intake/Exhaust valves
113 Lobe Separation Angle (LSA)


Your cam could be completely different as there are 100 different cam choices. The above would be a good base line guess if I had to use some kind of cam specs.


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## adamrysz87 (Nov 14, 2021)

Hi Jim, You helped me once again, You are great !
Could you please help me finish spec? I am not sure about a few things I marked in red.










My exhaust manifold is numbered 9777648-LH2. Is it a short style?
I also took a picture of the exhaust, is it H-Typ?


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## PontiacJim (Dec 29, 2012)

NOTES - Write "Stroker Kit - 455CI" He will know what that means. The carb will be jetted and adjusted for a 455.

OK, Year - looks like it refers to heads, so 1968.

Under Installed - I see STR. That box may also have ADV/RET which means how the cam is installed - STR - Straight Up, ADV - advanced, RET -retarded. This is used for drag racers. So just go with the STR and leave the other box blank because you did not advance or retard the cam when it was installed.
Ported - No
Exhaust Manifold - Cast Iron, *not short.*
Primary Tube Dia. - write NA (Not applicable, you don't have headers)
Collector Dia & Brand - write NA
Style - Ram Air III
Full Exhaust - Yes
Pipe Dia - 2.5"

You have the "H" pipe - that is the crossover tube that connects the dual exahust. So in the box for "NOTES" on the form, write in "Dual Exhaust, H-pipe, High-Performance Mufflers"

Axle Gears - This is the car's rear end gear ratio, 3.08, 3.36, 3.55, or??
Altitude Range - how high you live above sea level. If you lived in the mountains, you could be 4,000 feet above sea level. The air is thinner/less dense, so a carburetor can be calibrated for the thinner air. So the question is asking how high above sea level (with sea level being 0 feet) your car will be driving around.


I think that covers it?


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## adamrysz87 (Nov 14, 2021)

Jim, 
SMI sent a reply that for this compression 10.25 and Iron Heads I need some type of octane boost or race and pump gas mixture.
Is it correct?

_,, I am guessing that is a stock cam? With 10.25 and iron heads, you're going to need some type of octane boost or race and pump gas mixture to avoid detonation. "_


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## Sick467 (Oct 29, 2019)

For those of us with the 10 plus compression Pontiacs...93 octane fuel is a minimum and some need to boost up the octane from there to avoid detonation. So, yes it's true that the higher compression iron headed Pontiacs need higher octane fuel than what is available at the pumps.


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## PontiacJim (Dec 29, 2012)

adamrysz87 said:


> Jim,
> SMI sent a reply that for this compression 10.25 and Iron Heads I need some type of octane boost or race and pump gas mixture.
> Is it correct?
> 
> _,, I am guessing that is a stock cam? With 10.25 and iron heads, you're going to need some type of octane boost or race and pump gas mixture to avoid detonation. "_



Genrally, yes, especially with the 1968 and up "open chamber" heads. It seems the 1967 and earlier "closed chamber" heads can take the higher compression better, but still may need an octane booster or racing gas blend.

If the engine experiences the "pinging" sound under harder acclerations or up hills, that is detonation. Detonation can destroy an engine - crack pistons and ring lands, damage head gaskets, and destroy rod bearings.


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## adamrysz87 (Nov 14, 2021)

OK, in Europe the standard is 95 or 98 octanes. Hope this fixes the problem.

Thanks !


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## Scott06 (May 6, 2020)

Octane ratings in Europe and US are different methods American vs European fuels – Octane rating – eTuners.
your 98 may need octane boost depending on timing etc.


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## adamrysz87 (Nov 14, 2021)

I didn't see about the octane difference ... Thanks for the clarification.


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## adamrysz87 (Nov 14, 2021)

Hello, I have a very strong pump mounted in the tank that does not fit my carburetor.

Holley® 12-302 - Muscle Car EFI Fuel Pump Module (carid.com)

What fuel pump should be for the Quadrajet 7042218 or SMI Rochester 4BBL Quadrajet?


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## adamrysz87 (Nov 14, 2021)

Hello, I have a very strong pump mounted in the tank that does not fit my carburetor.

Holley® 12-302 - Muscle Car EFI Fuel Pump Module (carid.com)

What fuel pump should be for the Quadrajet 7042218 or SMI Rochester 4BBL Quadrajet?


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## PontiacJim (Dec 29, 2012)

adamrysz87 said:


> Hello, I have a very strong pump mounted in the tank that does not fit my carburetor.
> 
> Holley® 12-302 - Muscle Car EFI Fuel Pump Module (carid.com)
> 
> What fuel pump should be for the Quadrajet 7042218 or SMI Rochester 4BBL Quadrajet?


You don't need to change anything. Get a fuel pressure regulator that will allow you to set the fuel pressure to about 5 pounds of pressure (US).


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## adamrysz87 (Nov 14, 2021)

Jim, is it possible to adjust the fuel pressure on the carburetor ?
There are two screws on the front, what are they for?


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## PontiacJim (Dec 29, 2012)

adamrysz87 said:


> Jim, is it possible to adjust the fuel pressure on the carburetor ?
> There are two screws on the front, what are they for?
> 
> View attachment 150772



No. Those are idle mixture screws. A pressure regulator is your only choice so you can adjust the fuel pressure going into the carb.


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