Carburetor clarification 1974 L82 Auto

[Petter T.]

Hi all, I have a 1974 L82 with Th400. The car is matching # and the carb is stamped 7044210 wich is correct.

The car however is running rough at idle and the carb is probably the culprit. Yesterday I ordered a replacement (not matching #) from national carburetors to get the car on the road. The guy who hjelped me at NC concluded that there was only one carb for the 1974 Corvette.

I pointed out that in the black book, 9 different Numbers are listed for the 1974 Corvette, and asked him to explain why GM would have 9 different numbers for the same carb. I did not get any explanation other than that ND4495 is the common carb, and that was what I would get...

My question to the experts here are as follows:

1. Is there only one carb for the 1974 Corvette?

2. If so, does this carb have the same jets, needles, spec. and tuning for the different engines and trans combos for 1974?

Br
Petter

[Michael W.]

Re: Carburetor clarification 1974 L82 Auto

1. no, there's nine but three of them are for big block engines.

2. no, each is different.

I wouldn't expect a guy answering a phone at a jobber to know any different. They list the quoted part number being correct for 41 different Chev products as well as 15 different GMC.

It might be much simpler to fix your existing carb.

[Petter T.]

Re: Carburetor clarification 1974 L82 Auto

Thanks Michael, You are confirming my suspicions...

National Carburetors claim that they are the best in the business - how can they not know this?

As for rebuilding my existing carb, I'm located in Norway, and sending my original carb means that it will take ages just to ship it forth and back.

Do You know where I can find the spec of the 7044210 carb?

Br
petter

[Leif A.]

Re: Carburetor clarification 1974 L82 Auto

Hilsener, Petter

This document from the GM Heritage Center should give you all the information you require.

https://www.gmheritagecenter.com/docs/gm-heritage-archive/vehicle-information-kits/Corvette/1974-Chevrolet-Corvette.pdf

[Petter T.]

Re: Carburetor clarification 1974 L82 Auto

Hei Leif.

I was thinking of internal carb spec...
Browsed through the document from the heritage center, but could not find this here.

Petter

[Joe L.]

Re: Carburetor clarification 1974 L82 Auto

Hi all, I have a 1974 L82 with Th400. The car is matching # and the carb is stamped 7044210 wich is correct.

The car however is running rough at idle and the carb is probably the culprit. Yesterday I ordered a replacement (not matching #) from national carburetors to get the car on the road. The guy who hjelped me at NC concluded that there was only one carb for the 1974 Corvette.

I pointed out that in the black book, 9 different Numbers are listed for the 1974 Corvette, and asked him to explain why GM would have 9 different numbers for the same carb. I did not get any explanation other than that ND4495 is the common carb, and that was what I would get...

My question to the experts here are as follows:

1. Is there only one carb for the 1974 Corvette?

2. If so, does this carb have the same jets, needles, spec. and tuning for the different engines and trans combos for 1974?

Br
Petter

Petter-------

The number 1 cause for the condition you describe is either a "heavy" float or leaking soft plugs in the bottom of the fuel bowl. Either of these is easily rectified. First of all, replace the float with a new nitrophyl float (as original). Second, seal the soft plugs using JB Weld. Third, rebuild the carburetor with a new kit.

All of the above referenced parts are GM discontinued. However, you can get what you need from several sources including www.cliffshighperformance.com

An excellent reference for carburetor rebuilding is:

"Rochester Carburetors" by Doug Roe and Bill Fisher published by HP Books.

[Leif A.]

Re: Carburetor clarification 1974 L82 Auto

Hei Leif.

I was thinking of internal carb spec...
Browsed through the document from the heritage center, but could not find this here.

Petter

what "specs" are you seeking?

[Petter T.]

Re: Carburetor clarification 1974 L82 Auto

Hi Joe.

I find the carburetors a bit intimidating...
But I guess that fixing it myself is plan B if the carb I get is all wrong.

Petter

[Joe L.]

Re: Carburetor clarification 1974 L82 Auto

Hi Joe.

I find the carburetors a bit intimidating...
But I guess that fixing it myself is plan B if the carb I get is all wrong.

Petter

Petter------


A Q-Jet is really not all that difficult to rebuild. It might SEEM intimidating but it's really not a big deal, at all. Obtaining the book I referenced will be a big help.

[Petter T.]

Re: Carburetor clarification 1974 L82 Auto

The size/weights of jets, needles... Adjustments
I'm a novise in this, but if I had the specs for the 7044210 I could get them to National Carburetors.

Petter

[Joe L.]

Re: Carburetor clarification 1974 L82 Auto

The size/weights of jets, needles... Adjustments
I'm a novise in this, but if I had the specs for the 7044210 I could get them to National Carburetors.

Petter

Petter-------


If you have the GM Service Manual for your car (which, I think for 1974 includes the overhaul manual), the carburetor specs should be in it.

[Petter T.]

Re: Carburetor clarification 1974 L82 Auto

Joe, I've just gone through the service manual - there are no specs. here, only adjustments - which however are more or less the same for all the 1974 carbs...

Petter

[Patrick H.]

Re: Carburetor clarification 1974 L82 Auto

Do you have any idea what the history is on the current carb? Odds are that it probably has the correct jets and rods in it.
As such, I wouldn't worry about getting new ones. They essentially last forever.

To replace the float, you don't even have to remove the carb from the car if you don't want to. I've done it without removing it more than once.
I would not only use the book Joe mentions but also check out YouTube for videos on how to do it.

I would recommend a new float, new air horn gasket, and new accelerator pump. Buy a few of the air horn (bowl cover) gaskets, as they seem to always tear when you remove them. Even if you do nothing else, there is a good chance that you will improve your performance by replacing these.

Needle & seat, fuel filter, other gaskets can be replaced as well, but are less likely to do as much.

Also, remember the old but oh-so-true saying...
The vast majority of "carb problems" are caused by your ignition.

[Mark E.]

Re: Carburetor clarification 1974 L82 Auto

Petter-------

The number 1 cause for the condition you describe is either a "heavy" float or leaking soft plugs in the bottom of the fuel bowl. Either of these is easily rectified. First of all, replace the float with a new nitrophyl float (as original). Second, seal the soft plugs using JB Weld. Third, rebuild the carburetor with a new kit.

All of the above referenced parts are GM discontinued. However, you can get what you need from several sources including www.cliffshighperformance.com

An excellent reference for carburetor rebuilding is:

"Rochester Carburetors" by Doug Roe and Bill Fisher published by HP Books.

As usual, Joe's recommendation is a good one. If the carb is original to the car, there's a good chance you don't need to worry about the specs for the jets, rods, secondary air door cam, etc. Unless someone changed them in the past, what you have is what came with the car.

Cliffhighperformance.com is probably the best Q-Jet resource in the US. He can rebuild it for you. If you choose to rebuild the carb. yourself, order a kit and float from him. This is also a good time to check the vacuum break; it's not uncommon for them to fail and cause a vacuum leak. In case you're suspicious about your carb's past, Cliff can also advise which jets, rods, vacuum break are original for your application.

In addition to Doug Roe's book that Joe recommended, you might also consider Cliff's book which is available on his website. It has a detailed step-by-step procedure with photos for rebuilding.

[Duke W.]

Re: Carburetor clarification 1974 L82 Auto

Petter-------


If you have the GM Service Manual for your car (which, I think for 1974 includes the overhaul manual), the carburetor specs should be in it.

The Chassis Service Manual should have specs, but I believe the '74 Chassis Overhaul manual is a separate book and it should have a detailed step by step procedure on how to disassemble and assemble the carb and all the checks and adjustments.

1974 was the last year before the nearly universal adoption of catalytic converters. Manufacturers struggled to meet emission standards with the crude early emission control add-ons, and 1974 was the worst year for driveability problems and complaints were rampant... everything from cold start and warm up to rough idle, poor acceleration, detonation... just about every complaint you can imagine.

The OP should use the manuals to check out all the emission control devices, and either make sure they are working or remove or disable them, convert to full time vacuum advance with a B28 VAC (because it's an automatic) and optimize the spark advance map. Idle speed should be set at 700-750 in Drive with the idle mixture screws set using the pre-emission control procedure. (Forget the "RPM drop" final adjustment.)

If cold starting and warm-up driveability are issues, there should be enough adjustment authority in the choke and fast idle systems to smooth things out. 1974 carburetors were calibrated very lean including cold start, warm-up, idle and cruise in order to meet emission standards. WOT calibration is probably okay because operation at WOT was not required during the emission certification test.

Duke