I needed some parts for the choke of the carb on my 1968 L79 so I decided to investigate which carb I had. Turns out its a Carter made Quadrajet stamped # 7040202. Based on the number, it is my understanding that its from a 1970 car with an automatic. My car is a 4 speed.

Are there any issues with continuing to use this carb or should I source the correct model? Or maybe my understanding of the # 7040202 is incorrect and I'm OK.

Thanks for the input!

 

midway through this link is a listing of Quad numbers, that number does not appear to be a Corvette number but if you have no problems there is no point in chasing down the correct one.

http://www.digitalcorvettes.com/foru...ad.php?t=88376

 

Yea looks like its from a Chevelle or Camaro. Runs fine so I guess I'll just go with it, just didn't know if anyone smarter than I knew if there was an issue running a carb that is specific for an auto trans.

 

If the 7040202 carb you have is still configured as it was from the factory, it has .076" main jets, .044" main rods, and BA secondary rods. The "correct" carb for your car should be a 7028207 which came with .071" jets, .046" rods and AN secondary rods.

Those differences will cause you to run very rich at 'cruise' and at 'WOT'. You would have to be the one to make judgement as to whether this is the case, or not. But, the basic carb will perform similarly; and, if rods and jets were changed to match the needs of your engine, the carb itself should be no problem for you.

There may be some other manufacturing differences, but I don't have enough knowledge to determine those. I suggest that you send an e-mail to Lars Grimsrud at V8FastCars@msn.com and ask him what changes you should make to configure the carb you presently have into a good match for your L-79 engine. If you have modified the engine or components attached to it (manifolds, cam, etc.), you should inform Lars of that so that he could factor in any modification required for the stock configuration of that carb.

Good luck.

 

Thank you 7T1Vette, that is exactly what I was looking for! I'll get a hold of Lars and see if he can help me. Thanks again!

 

If your car does not seem to be running too rich it may be OK. A lot of us have had to richen up to compensate for today's crappy gas.

 

Appreciate all of the input on this, I did get a hold of Lars and he was of the same sentiment as you DocHorsepower. Just use it, there should be no issue and no need to re-jet the 1970 carb. Also, the only noted difference between the AT and MT versions is the mounting provision for the kickdown linkage on the AT.

 

You and I have discussed this in the past, you can't compare carb calibrations based on rods and jets alone. There are far more components playing a role here. 7040202 is likely running a leaner air bleed (larger) vs the 7028207. The difference in size of the cruise circuit (a 76/44 has a much larger cross section vs a 71/46 on simple evaluation at cruise), but that could also be accounted for in APT or power piston pin adjustment.

Aside from that; you should never attempt to "create" a different carb by copying jets from one carb number to another. You need to start with the baseline that carb number has... assuming it's unmolested. If it has been molested, there's a whole lot more work to do.

I echo the sentiment others have said; if the carb runs well and matching numbers are not a concern, you're at a good start.

7t1's concerns about richness are not completely unfounded though, there's a lot more adjustment on the "rich" side of tuning vs the lean side before you have drivability issues, so it may be well worth the time spent to do a little tuning. However, I would do it in small increments from your current baseline. Lars has a good procedure in his papers that will get you very close if you have some time, patience, a stop watch and a small test strip.

Good luck!

 

Shark Racer...

Uhhh.... that's why a suggested he contact Lars.

Why don't you just post info for the OP instead of trying to 'one-up' other folks just trying to help him????

 

There is info in addition to my correction of the info you posted.

Some of the info you presented is dangerous. Trying to install a leaner calibration from one carb into another could burn up an engine.

Talking to Lars is a very good choice, and I would never contradict you on that.

 

The man asked what the differences were. I gave him the information asked. I then indicated that if rods and jets "were changed to match the needs of your engine" the carb might run better. I DID NOT tell him to swap over to what the stock carb had.

As I mentioned before, maybe you should just give YOUR advice to the OP rather than trying to 'correct' other posts.

 

You told him he would run "very rich", this is not necessarily true. You are absolutely correct in telling him to use his own judgment and try for himself.

Given that he has come to this forum asking for information on how a wrong carb might impact his motor, I would not want him to begin his judgement thinking he was running "very rich".

Also, the information is there for you as well; you should know by now that there is more to a calibration than simply jets and rods, and Lars himself has corrected you on this in the past. Just because the OP asked a question doesn't mean that others in the conversation aren't allowed to read and learn. If I give less than perfect advice, I welcome people to correct me. I do not and can not know everything and welcome the opportunity to learn more.

In fact, even the things I've mentioned are just barely scratching the surface; cam timing, compression ratio and others can also impact the calibration of a carburetor, particularly the very sensitive QuadraJets.

 

You can argue all you want, but it's just more BLAH, BLAH, BLAH...

You have this 'thing' where you just have to prove that you're right [whether you are or whether you aren't]. I'm sure that you have a pretty fair working knowledge of YOUR car[s]. Maybe you should just limit your banter to sharing your opinions with the OPs.

 

The OP did contact me, and he should be on his way to cruising happiness...

For info, this is my response to him on the re-jetting issue, where people try to "create" a carb by using the jetting specs from a different carb:

"David -
If the '70 carb is in good shape, I'd keep it and run it. It's a better carb than the '68. No, you cannot re-jet the '70 carb to the '68 specs - that makes as much sense as running Stromberg carb jet sizes in a Holley because you're running the Holley on a '49 Cadillac that came with a Stromberg. You run the jetting for the carb number - not for the car year. The auto trans carb has provision on the throttle linkage for the kickdown cable - there are no other differences between the AT and MT carbs.
Lars"

Basically, there is more to jetting than just the jet and rod sizes: The air bleeds play a bigger role in determining fuel mixture than the jets do, and in the '68 carb, the air bleeds are super-restricted with the brass inserts. The '70 carb has no air bleed restrictors at all - it runs the casting holes wide open to really lean things out with the bigger jets. Swapping jetting specs between these two carbs would really screw things up... always run the jetting specs for the carb number as a baseline, and tune the carb from that baseline based on what the engine/car needs and observed objective results.

Lars

 

What jets/rods did you end up recommending for the OP? Or did you recommend to use the carb "as is"?