I've been reading a lot regarding the q.jet carb. I also see that one came oem on 307's. It is well supported with aftermarket parts, jet kits and all, and is well liked and respected.

What caught my eye was the small primary venturi! I would imagine that around town with the primaries I might bump up my mpg by a few, hopefully.

Will one of them fit a C style intake? I thought it would be a fun project to find one for a 307 at the j/y and rebuild and try to set it up for the 300. They don't seem too complicated. I'm aware of the problem with the oem leaking plugs, and the possibility of leaking t.shafts, but all this is addressed by the aftermarket.

And, if it doesn't work, I can use the secondary flaps for frisbee golf.

Thoughts, anyone?

 

I know someone that runs a smaller one on a chevy 305 and it does well not sure if they make a adapter or not though that would fit an offenhauser intake though. It may be easier to find one from a 305 considering alot of people don't care to build them.

 

You would need an adapter to fit the carb onto a holley type flange intake. The bolt pattern is different, but most intakes have the dual bolt pattern. The main reason for needing the adapter is to clear the secondaries and seal the baseplate.

 

Edelbrock, TransDapt, etc., make a spread bore/square bore adapter. Lots of small block V8s and V6s in marine applications with Qjets. If you can't readily come by a Qjet, Holley makes the 80555C model spread bore 650 cfm as a Qjet replacement. Small primary bores with the larger toilet-flusher secondaries but not as large as the Qjet secondaries.

 

I have read that guys recommend port matching the adapter to the Q.jet because the secondaries make a hard turn. Does that mean opening up the adapter the size of the secondaries? Of course that is for the 'C' style intake only.

But if one left the adapter as is and put on a DP intake, what would be the effect of funneling it down like that?

 

Not really a hard turn but a connecting of mismatched bores done in probably the most economical manner as far as casting and machining go. For a C manifold I suppose one could remove all the material separating the bores essentially making it an open spacer leaving the adaptive bolt hole configuration for mounting the carb to the manifold. Next time you are by a performance parts house take a look at one.

 

What is the use of having the large secondaries if they are going to be funneled down? What is the effect of doing that?

 

The secondary bores on the Qjet are quite a bit larger than the bores on a standard 4-hole manifold. On OEM Qjet installs the manifold and carb were matched. If one were to use a square bore carb on a spread bore manifold the adapter is required and is mounted upside down from what it would be if installing a Qjet on a square bore manifold. On an open manifold I think the only reason to have the adapter is to adapt the carb bolt holes to the manifold bolt holes thus the material making up the bores in the adapter could conceivably be removed to make it open while retaining the adaptive bolt-hole function. Or there might be an adapter plate available that does the same thing. At any rate on our engines I don't think it would make a difference...the engine is not going to use the full volume available through the Qjet secondaries anyway and the funneling down might not have any impact at all.

 

I will be picking up the Q.jet soon. The more I read the more interesting and intriguing the project becomes. What I am surprised by though, is the cost of the a.market kits, etc., for the Q.J. $150 for the calibration kit alone!

 

You think coming off of a chevy they'd be cheap...

 

All the Chevy stuff I have seen half the price of the equivalent ford stuff, and available in twice the retailers. GM made millions of these carbs, I would think that parts would be cheap. But they are not.

I found that even the Q.J.'s that came oem on 305's were 750 cfm. But GM altered the movement of the air flap to make it on 575 cfm.

 

I am having enough fun trying to tune my old carter. I have seen what happens when things go wrong on a Q- jet and one loses their mind. So I am curious to see how it works out lol.

 

Interesting info: The Q-Jet takes these design constraints or opportunities to almost an extreme solution by using a super small primary venturi to speed the air flow to a point where it applies so much shear that the flow tears the molecules apart which emulates the phase change of liquid fuel to gaseous which is the state that burns.

Typically the OEMs augment this with an exhaust heated hot spot under the plenum to supply enough heat to force the phase change. Hot rodders eschew this method because it also heats the incoming air and resulting mixture. The result of heating the mixture is to reduce its density, hence engine power and increases the engine's sensitivity to detonation. The problem is the difference in mixture velocities between street engines and race engines. On the street an engine seldom sees the kind of RPM that results in enough air flow speed through the ventuires to shear the fuel into a combustible mist, where race engines tend to operate all the time in RPM ranges that can do this. The difference between these operating parameters is why the factory heats the intake mixture and why the Q-Jet more than any other common 4 barrel out there does such a good job at maximizing fuel mileage.

One of the biggest and most common prob with the QJ seems to be vacuum leaking throttle shafts. Impossible to tune if that is the case. There is also a couple of oem plugs inside that wear and leak, kill mpg. Easy fixes.

 

Cliffs High Performance Quadrajets :: Quadrajet Rebuilding | Quadrajet Rebuild Kits, Bushing Kits and Parts and Carburetor Restoration

He has the throttle shaft bushing kits IIRC.

 

You know, just looking around and pricing parts for the rebuild, with a calibration kit and adapter, I'm looking at well over $300 easy! For a thirty-five-year-old carb, and that is with me doing the work!

I am really surprised how pricey the parts are for a carb that sold millions. Also, considering how many people hate the Q.J., that makes me hesitant. I'm not a carb expert. I'm just a hard-working guy trying to find a carb that will give performance and save me some money on mpg around town too. I'm not so sure I want to toss away $300-$400 on a carb that may be beyond my conprehension or mechanical abilities.