I am looking for a non-CCC Q-jet for my 305. Is there any particular year or model i should look for? I know the 750cfm model will suffice. The only vacuum ports i use are for the PCV and the Dist advance.

 

That 305 will never fully use a 750 CFM carb. Any Q-Jet from 1980 and earlier Chevy 350s will be a good place to start. Buick, Olds, Pontiac, and some Cadillac Q-Jets have forward-facing fuel filters which would force you to bend your existing fuel pipe to accomodate it; a real PITA.

Here's a formula you might find useful for determining what size carb you need:
(engine CID x RPM x %volumetric efficiency)/3456

Assuming a 1.5" vacuum drop across the carb and 100% volumetric efficiency, your 305 at 5000 RPM would need only 442 CFM! Your average 600CFM Q-Jet is more than capable of handling the average 305.

 

i thought that qjets only came in 750 and 800 cfm and that the secondary spring controlled the real world CFM (?)
I am going to a swap meet next month and was wondering if there is anything specific i should look for. Thanks for the tip on the fuel inlet
sean

 

Most Q-Jets are 750 but, as I understand it, late model ECM Q-Jets were "fixed" to 600 CFM for small block V8s and certain V6's like the 4.1 Buick.

Rochester did make some 850 Q-Jets but the only place I've seen them is on the old Cadillace 472 and 500s and on some Olds 455-powered GMC motorhomes. Talk about power, a 1970 Eldorado came with a 500 CID monster that pumped out 375HP and 520 ft.lbs. of torque! It put the power to the road and your back hard against the seat without breaking a sweat (God, I MUST be old for remembering such a car!).

 

I remember the 500's too, dont feel bad!
I was curious how much the qjets changed from the 60's until they went computerized, and if the later ones are more efficient?

 

Actually the 850 CFM Q-jets were on alot of the later model cars before they went to the computer controlled version. The computer controlled versions are all 850's but the rear flaps just don't open all the way. You would be supprised what you will find the 850 CFM Q-jets on. The way to tell the 750 CFM from the 850 CFM is by looking at the size of the primary venturies.. the 750 will be smaller than the 850.

 

The 750 and 800 CFM versions were the most popular. The chances of finding one of the super high flowing units is very remote.

That leaves you with the 750 and 800 CFM units in the real world. THe only diff between them is the size of the primary venturis. It's tough to tell a few 32'nds of an inch difference in venturi diameter by eye so here's the easy way: The main venturi (not the booster venturi out on the end of the stalk) which is cast into the carb's main body will be perfectly round all the way around it's circumference on a 750 CFM model. The 800 CFM model will have a little "lump" on one side of the venturi- you can't miss it once you are looking for it.

The lower flowing QJets are just versions of the basic 750 or 800 CFM models but with secondary air doors that don't open all the way. Modifying a low-flow QJet so it's secondary air doors open all the way (85* or so) will gain you back that CFM.

Since the QJet is a vacuum secondary carb you can put it on a little 305 with no chance of over-carbureting. The secondary air valve senses air flow and only gives the engine what it can handle. GM has been using them on very mild 305s for a long time with no problems.

What you REALLY want to look for: LITTLE OR NO EMISSIONS CRAP ON THE CARB! Get as simple a QJet as you can find. There are all kinds of external gee-gaws that the factory added on at various times to improve emissions like idle compensators, secondary choke pull-offs, ALTITUDE COMPENSATORS BELIEVE IT OR NOT! The list goes on. Most TRUCK carbs are thankfully more free of emissions crapola than their car counterparts. Edlebrock also makes NEW QJets using Rochester's original tooling that they bought a few years ago- they are performnace oriented in their calibration and make good power right out of the box.