Which would be a better choice... a pre-'74 Q-jet, or a '75-'80 Q-jet?

I have access to a '69, '70, and a '79 carb... all from a SBC application. Which would you use? I am leaning twords the later carb, as it seems to have less wear-and-tear. I've also read that the later carbs had better quality castings, and did not have as many "leaker" problems as the earlier carbs.

Lets hear from you Q-jet experts!

 

I make no claim to be an "expert" about anything, but I can spout off an opinion....

As far as which one will run better, anything pre-71 will run better than anything 71-up, due to the leanness of the later carbs' idle and transfer calibration for emissions reasons. The mid-late 70s ones were the worst.

Alot of the casting issues and leaks can be repaired with little effort.

IMHO the all-around best core to build from is 7029202. From a 69 350 with double-hump heads.

 

Thanks RB... anyone else?

BTW… I have a 7029202 from a 255hp 350. The stock engine had 441’s, not double hump heads. My other core is a 7040202 from a ’70 300hp/350. I’m still wondering if I can make the later carb work, since my two older cores are kind of rough.

 

I'd say anything post '75 is better because most of them are 800 CFM as opposed to 750s and weren't as prone to the air horns being warped. The lean calibration is just that, a calibration issue and can be corrected using jetting/rod changes and manipulating the APT setting. Plus they are ideal for electric choke conversions usinf any switched 12V source and they are much more plentiful.

 

I could be wrong but I thought that the later q-jets had an extra adjustment for part throttle or something that the older ones didnt.

Ben

 

Yes, That's the Adjustable Part Throttle(APT) stop. Some were adjustable via a plugged off scew in the baseplate between the idle air adjustment screws. It worked by screwing in or out a screw that biased against a hairpin looking spring and the power piston had an extension pin on the bottom that the piston rides on.
The second and later style used a threaded stop that was parrallel to the power piston bore,The powere piston had a horizontal pin and when the piston is being pulled down in it's bore the pin on it would stop on the APT screw. This was accessed via a plug in the air horn near the bowl vent. Emissions dictated that cars needed to be lean in the idle circuit and to over come the stumble from the transition to the power circuit the APT would start to "tip in" the power circuit earlier, and also aids in overcoming a lean condition the EGR creates at cruise speeds.
For what it's worth I leave them alone at all costs unless it is clear that I can't adjust out a stumble. Parts store rebuild carbs usually have the pin on the earlier power pistons clipped off to completely disable this adjustment. I've never seen a second design APT carb have it's APT disabled like that.