Choosing a new carburetor? - Impala Tech
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post #1 of 11 (permalink) Old 06-29-2015, 03:32 PM Thread Starter
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Choosing a new carburetor?

So after doing everything to try and fix my lack of power problem, I've decided that the carburetor is probably dirty and gummed up. I re-tuned it last week and performance improved but I still felt some sputtering and hesitation, especially on hills. It might need more adjusting and I'm probably going to take the car to get a check up at my mechanic before I do anything. But I want to prepare ahead of time if I decide to get a new carburetor.

My uncle who is a mechanic down in southern WA offered to rebuild and clean my 4-barrel quadrajet (that is the original carb since 1967 and I'm pretty sure it's never been rebuilt and maybe not even cleaned). But he also said that I would be better off just getting a new one since they aren't that expensive and I would save a lot of time instead of dealing with the old one. He specifically mentioned Edelbrock as a recommended brand for a new carburetor. What is everybody's thoughts on if I should rebuild mine or just get a new one?

Right now I'm at Summit narrowing my results down on Edelbrock carbs: http://www.summitracing.com/search/d...rder=Ascending
I honestly don't know what to choose for CFM and don't know what I have right now or what my engine spec CFM is. I would appreciate more performance, maybe even a maximum performance carb but I'm also worried that it would suck up more gasoline and become a gas guzzler. Any suggestions?

1967 Chevrolet Impala 4 door "post" sedan
327 5.4L V8 - 3-speed TH350
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post #2 of 11 (permalink) Old 06-29-2015, 04:29 PM
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Spec cfm is a 630; though it only flows that at WOT. If you don't have a motor to suck that much it won't flow that much as the Rochester QuadraJet only flows what the engine can use. It is vacuum actuated so if there isn't any vacuum to pull the doors open then the carb is a just a small two barrel with the rest of the carb held in reserve until you need it.

The Edelbrock carburetor is actually a Carter AFB design manufactured under license which has four fixed venturis similar to a Holley. Carter also made an AVS model (available used, or remanufactured that is also a square bore carb which is to say that all for barrels are equal spaced) that uses the same variable venturi technology (it has no fixed cfm size that has an upper limit but will flow only what you need). To change over to a square bore carb like the Carter/Edelbrock or Holley carb you will need a matching manifold to bolt it onto. Your current Rochester is a spread bore that has two man hole size rear barrels and two tiny front primary barrels.

Here is a picture of a Rochester QuadraJet intake manifold:



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post #3 of 11 (permalink) Old 06-29-2015, 06:02 PM Thread Starter
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Thanks Dave! You've always been a walking database.

I actually think I might be leaning towards a rebuild of my Q-jet. It might be the cheaper option and I don't think anything that I buy will live up to the Q-jet.

1967 Chevrolet Impala 4 door "post" sedan
327 5.4L V8 - 3-speed TH350
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post #4 of 11 (permalink) Old 06-29-2015, 10:49 PM
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The Q-jet is a great carb, stick with it. The Edelbrock will probably give you worse MPGs.
I would also lean towards rebuilding what you have. There were lots of different models of the Q-jet. Yours is sized and tuned for your engine.



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post #5 of 11 (permalink) Old 06-29-2015, 11:28 PM Thread Starter
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Yeah I'll talk to my uncle again about it. There is also the option of sending it to an "online service". Meaning there's carburetor rebuilding services online. My brother did the same thing with his Buell carburetor to get it cleaned and rebuilt. His motorcycle sounded crisp, clear, and ran better then ever after doing it.

Here are ones I found that also advertise their services on ebay with positive reviews: http://carbjunkys.com/ and http://www.coastalcarburetor.com/. Costal Carburetor is offering $185 for a rebuild. I'll have to contact Carbjunkys for a quote.

1967 Chevrolet Impala 4 door "post" sedan
327 5.4L V8 - 3-speed TH350
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post #6 of 11 (permalink) Old 07-01-2015, 08:26 AM
 
 
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If it's not really a 100% restoration find a later model Qjet that you can have an electric choke setup, & have that rebuilt.

Jim

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post #7 of 11 (permalink) Old 07-03-2015, 07:34 AM
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If you get the edelbrock you will need one of their fuel pumps.
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post #8 of 11 (permalink) Old 07-08-2015, 10:17 PM Thread Starter
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Quote:
Originally Posted by richards69impala View Post
If you get the edelbrock you will need one of their fuel pumps.
That's good to know. I just installed a new fuel pump that isn't a Edelbrock.

1967 Chevrolet Impala 4 door "post" sedan
327 5.4L V8 - 3-speed TH350
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post #9 of 11 (permalink) Old 07-09-2015, 10:02 AM
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Originally Posted by richards69impala View Post
If you get the edelbrock you will need one of their fuel pumps.
Why??

carburetor doesn't care what pump delivers the fuel and pump doesn't care to which carb it's delivering. As long as the pressure remains around 6.5 psi, it doesn't matter.
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post #10 of 11 (permalink) Old 07-09-2015, 11:24 AM
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I was assuming the Edelbrock carb has a lower MAXIMUM fuel pressure at the inlet. The MAXIMUM fuel inlet pressure with a Holley 4150 and 4500 series carb is seven pounds per square inch. I set my adjustable pressure regulators for only FIVE psi not seven at the inlet of my Holley 1150 cfm 4500 series Dominator carb. The maximum fuel pressure is a function of the float's buoyancy and the length of the lever arm. A bigger float can generate more lift but it occupies more volume best used by fuel in a carburetor.

You want volume not pressure at the fuel inlet. I run a half inch line with a three eighths return line to obtain the volume of gas that a 750 horse normally aspirated big block requires. A carburetor has zero gauge pressure inside the fuel bowls (one atmosphere pressure). You do not need any more pressure at the inlet than would be required to pump gas through the small diameter of the seat that the needle is pushed against by the pressure of the float on the end of a lever arm. I also replace the gas needle and seat with the larger diameter hole used with an alcohol based carb from Holley to increase fuel flow through the carb.

With my Aeromotive A1000 electric fuel pump I have to turn the pressure at the carb down from 46 psi. Because a three eighths fuel line is sufficient to return the unburnt fuel to the tank under the amount of pressure my pump puts out I do not stress anything (heating the pump, aerating the fuel in the fuel cell or running out of gas).

A stock mechanical fuel pump out puts less than five psi at WOT. It is designed to handle only 350 horsepower in terms of fuel volume. The fuel pressure is constant as it is supplied by a spring, but as with valve springs they have harmonics and can vary the load if the pump encounters a harmonic in normal operation. Problem is "normal" for you and what the engineers had in mind back in 1953 when it was designed may differ.

Aftermarket pumps are plumbed differently internally. They have up to five separate reed valves to move fuel from the low pressure side of the pump to the high pressure side to increase the fuel volume as well as having larger inlets and outlets than stock pumps. The stock pump has only a single reed valve to allow fuel flow and that is the biggest restriction in it's design. The aftermarket mechanical fuel pumps consisting of Edelbrock, Carter, Holley and others also out puts fuel at a higher 6.5 psi instead of the more common 5 psi used in stock service part replacement mechanical fuel pumps.

The BBC fuel pump and some high horse small block fuel pumps from Chevy had a return line built into the base of their mechanical fuel pumps to optimize fuel volume. As soon as the pressure reached it's preset 6 to 6.5 psi a valve opened returning the unneeded fuel to the tank.

Otherwise I agree; all fuel pumps are the same.

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post #11 of 11 (permalink) Old 07-10-2015, 09:34 PM
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My 72 c10 was having a problem restarting after I put on an edelbrock carb.Put their fuel pump on and problem went away.I was told they don't like to much psi at idle.I am by no way carb expert.I wish I would have just kept my holley carb on my truck.
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