I am wondering what might be the biggest difference between a 1970 4 speed Quadra jet and a 1970 automatic Quadra jet ?I have an opportunity to get a 7040251-0790 from a friend and it’s supposed to be all original . I was wondering how it would do on my 455?the carb I have now is the original 1974 toronado auto quadrajet and works pretty well, I rebuilt it myself and didn’t change any jetting . Would this 70 carb tend to run richer? My 455 was built pretty close to 70 specs with the exception of the carb and it runs a tad on the rich side .thank you

 

If I remember correctly, the automatics had a primary power piston and metering rods. The SMT had smaller jets and NO power piston or primary metering rods. Low speed throttle response might be affected with no power piston/metering rods. Joe P. will have some wisdom.

 

Your first problem is believing the myth that odd = manual trans and even = automatic. The '251 was used on every Olds full size with a 455 in 1970, and every one of those came with an automatic trans. It was also used on Cutlii with 455s.

Both the 7040251 and the 7043252 (yes the same number was used on Toronados for both 1973 and 74 model years) use the same 0.070" primary jets
'0251 uses 49B primary rods, '3252 uses leaner 53E primary rods
'0251 uses AU secondary rods. '3252 uses P/N 7045777 secondary rods. I cannot find any info on them.

 

All Qjets have power pistons and primary rods EXCEPT the 7040256 and '0255 carbs used on the W30 (SMT) and W31 motors. Note that ALL W31s (MT and AT) came with the 308/308 cam and thus used the Qjet without a power piston or rods.

 

well I did read that on the internet so!! Basically what you saying is the carb I have now would be leaner than the 70 ? I guess the only way to know for sure is to see what’s actually in the 70 carb and go from there .

 

Also 7040255 I believe on 70 W31.

 

Yeah, sorry about that. '255 is the W31 carb, not '257. My mistake. I picked the one on the wrong side of the '256.

I corrected my prior post to avoid confusion. Thanks for catching that.

 

Other things to consider, % of ethanol in pump gas, engine timing, maybe altitude. Also who's had there hands on the carb in the past. I wouldn't run a 50 yr. old carb without replacing and re-drilling the idle tubes.

 

as it turns out the carb was rebuilt and ready to run the reason he’s getting rid of it is because he bought the correct Pontiac carb for his gto . I know nothing about the full scope of work that was done to it .

 

Some things I learned

Start here, find the carb jets and metering rods list.

https://oldsjunction.classicoldsmobi...sfaq/ofcrb.htm

Notice how in 1968 they started to lean the carbs out by putting in a "49" primary rod. In 1967 most had a '71 Jet and a '44 Primary rod.

my "69 400 was running what I thought was a bit warm.. Did a lot of checks concerning cooling(re-cored original brass radiator) and timing( replaced a non-working vacuum advance, as well as check of mechanical curve) but during the troubleshooting I got my original carb rebuilt by Cliff Ruggles of Cliff's High Performance. He told me the ethanol gas runs leaner and he would put in his full taper '44 Primary Rods(Think 1967 emissions from the list) rest was stock 1969 '70 Jet and AU Secondary rods. Let me tell you, what an improvement. No more pinging! Restarts when hot, right up! 100F temp outside, engine runs 205F with 3.91 rear end and 195F thermostat at 70 mph.

A key assumption is that the air bleeds, other internal specs, etc. are the same between different year Olds Carbs, not just jet, and rod changes.. I do not know.

Also learned that the Quadrajet was a "lean" carb by design for fuel mileage.

Also found some race gas has ethanol to boost the octane, 100 Octane Sunoco Unleaded 260 GT is almost 10% ethanol

https://www.sunocoracefuels.com/fuel/260-gt

I can only tell you what seems to be working for me.

Good Luck!

Fred

 

Thanks Joe P. I knew that, I just forgot to add the important details.

 

since nobody else mentioned it ,another thing to consider is the Toronado carb's cable attachment is different than the Cutlass/442 and full size cars and will require modification if it matters to you, I would stick with the correct carb for your car