Alright guys, I'm getting really really really fed up with Edelbroke carbs. I haven't had any luck with this or any of the other ones that i've used on any of my cars. That being said.... I have no money to spend on a $300-$500 carb so i got to looking in our carb stash and found three, god i hate to say it, quadrajets. I'll pick the best one out tomorrow. It will be going on a moderately hopped up 300, it has an Offy 4 barrel intake, CamResearch CR266 cam, head shaved .040, and has been bored .040. I did some research and have decided that because of the very small primaries and the massive secondaries that this would have a pretty good balance of torque and hp. I'm guessing the ones i have are 750 cfm seeing as though that was the most common size. I have faith that it'll work pretty well, but i have honestly no clue. I know i'm committing a massive sin against all mankind, but if it works, it works lol. I'll let you guys know if this works well or not. Don't hate, i strongly dislike mixing Ford and Chevy, but i just cant afford a new carb and cant get the Edelbroke to work on anything so i'm gonna do what i have to so my truck will run right. If anyone else has committed such heresy, let me know how it worked please lol.

 

Sounds like a pretty cool project! And no, you're not the first:

https://www.ford-trucks.com/forums/6...n-a-300-a.html

https://fordsix.com/viewtopic.php?t=67681

https://www.ford-trucks.com/forums/1...et-anyone.html

In all reality, a Quadrajet is not a GM carb. It's a Rochester. GM just happened to use them. Just like Holley isn't a Ford carb, although that's what came on Ford engines.
Even if it WAS, the QJ is a good carb. They just have a bad rap due to some of the (fixable) issues they had.

Hope it works out great!

 

I don't have an issue with using a Rochester carb as much as I see issues with one that flows 750 cfm, which is roughly twice what a 300 needs. With that big a carb the secondaries will probably not start to open on a WOT accel until about 4000 - 5000 rpm.
There is a fellow - Tom Lowe - who makes three barrels out of Holley four barrels by blocking off one of the primaries completely. That is an interesting compromise. That would take a 600 vacuum secondary down to around 450 cfm - about perfect.

 

If I recall correctly, the Quadrajet only came in 750 and 800cfm configurations. It was the tiny primaries that allowed them to go on a wide array of engine sizes from the low 300s to mid 400s (cubic inches)

I wouldn't worry about them being too big. I've even run a squarebore 600cfm Holley on mine for a number of years, and it drove quite well (with MUCH larger primaries than that QJ would have). I could tell my throttle wasn't as "crisp" as it could be, but it drove just fine. I also got my best gas mileage with it (19mpg on a few trips and 15mpg around town).

Technically, a 600cfm carb would have 300cfm primaries.
Cfm requirements are:
CFM = (RPM * Displacement) / 3456
So, RPM is:
RPM = (CFM * 3456) / Displacement

With 300cfm primaries, that's 3456 RPMs where the secondaries need to start opening.

On a quadrajet, they're MUCH smaller, probably around 150 - 200cfm, which would mean the secondaries would start opening around 2000 - 2500 RPMs, which seems just about right. They'd never open up all the way, but they probably didn't on most engines they were installed on from the factory anyway.

 

I'm not suggesting it wouldn't run. With those small primaries it will work OK. But the secondaries will be largely wasted, so why not just run a 2150 2V? AB, Does your equation for flow take into account that the volumetric efficiency for the 300 is well below 100%? Maybe its 80% at the torque peak and probably below 70% - I can't remember - at the HP peak. So now it becomes 3456/.70 at the HP peak or more like 4937 RPM for a square bore 600. Somewhat better with aftermarket stuff but still largely unusable cfm capability.

 

What exactly is the issue you are having with the Edelbrocks? They are a pretty simple & reliable carb.

They can be sensitive to fuel pressure. So I always run a fuel pressure regulator. Even with a mechanical fuel pump.

 

My decently build 300 has a 625cfm street demon on it now. The primaries are about 1/3 of the cfm. The primaries are great while cruising of course but I don't even think about opening the other half until 2500 or so. A few years if tuning with an AFR gauge has gotten it pretty good but honestly the secondaries are too much. Stick with 500cfm and under for sure.

 

I'm just meaning that, if the 300cfm primaries on a 600cfm Holley are "decent", then the Quadrajet is going to be just fine.
The primaries on a QJ are around 180 - 220cfm, which is more akin to the size of the Holley 390cfm's primaries. Then, you have all the secondaries you want. Who cares if you don't use it all?

 

My thoughts exactly! It's there when you need it and you can say you have it when you dont.

 

I've done a little dynamometer testing on throttle bodies that were locked partially open. Imagine a secondary throttle plate at - say 45 - degrees with fuel being introduced above it. High speed filming and individual cylinder air / fuel measurements showed fuel impinging on the top of the partially opened throttle plate and slithering who-knows-where. Actually, we saw it get unevenly distributed off the bottom of the plate to the cylinders with as much as a five air/fuel ratio cyl-to-cyl spread. So given the option I would prefer throttle plate to be at or near fully open at WOT. Will it run with partially opened secondaries? Sure. Optimal? Nope. Just sayin'.

 

Why is everyone running a 600 and larger carb on a little 300 six?
Why not run a 390 cfm v4 Holley? And you may even get the 2nd half to open all the way!
Dave ----

 

Little 6?? You do know the 300 has the same cylinder size as a 400 V8 right?

The 600's are cheap and readily available. People are just about giving them away used. And for some reason they seem easier to get running good. I've never had any issue getting a 300 to run good with a 500-600cfm 4V carb.

I've never liked the 390 cfm 4V on a 300. They can be a royal PITA to get running good. They seem way more troublesome then the larger 4V. Many years ago I ran the 390 4V on a 300. I was never happy with the performance. I swapped over to a 500 cfm 4V and was much happier. I've owned a few 390's. The only time I was really happy with one was when I ran it on a 2.3L OHC 4 cylinder in a Mustang II I use to own. I don't know too many people that have ran both a 390 & a 500 or 600cfrm 4V and preferred the 390. Usually the guys that like the 390's haven't run a 500 or 600 on a 300.

Now I'd rather run a large 2V over the 390 cfm 4V any day of the week! And 2-2V carbs is even better! The 2-2V is about the best set up I've ran on a 240/300.

 

Look down the throat of the primaries. If you see a bump on the side of the bores, then it is an 800cfm model. Don't sweat the diff. The nice thing about the Qjet is that it is an adjustable cfm carb. You can adjust the air flap of the secondaries, making them open early or late. And, when the big toilet flusher secondaries (barrowing a term from Harte3) they are not thrown wide, but open according to how much the engine is drawing. Adjustable cfm.

If I remember correctly, and I don't feel like looking up the research to get the exact numbers, but the primaries flow just under 200cfm, iirc. I have run both the 750 and the 800 on my 300. If I were you, I'd check the vertical serial # behind the throttle. You'll be better off with one that begins with 1705. That means it is one of the last models, the end product of engineering evolution, right before they went electric, and made from 1975-1980. That model will not have the leaky bowl plugs. You can google the #'s and find out what it came oem on, and that will help you set it up when rebuilding. See, qjets are turned/were tuned in the factory for diff. applications by swapping air bleed restrictions, jets, m.rods, springs, etc. GM put a 750cfm qjet on everything from a 230ci engine to a 500ci caddy monster.

There is a great forum that will help a lot. Google Cliff Ruggles and that will get you to the forum. The Qjet is a good carb, but if you go half way on the rebuild, you're asking for trouble. Word to the wise: Because each qjet is tuned precisely to a particular engine, try to find one that came to an engine close in displacement to yours. That will make it easier to tune. Also, stay away from the qjets that have stickers from the mass rebuilders on them. Usually, those shops mix and match components, making turning nearly impossible for a mere mortal. After having spent more than a year rebuilding, changing air bleeds, jets, springs, metering rods, air bleed restrictions, enlaring air bleeds, taking the carb off and reinstalling, I would say this: Use the Summit or paypal credit option and buy a nice used holley or the Summit 500 cfm 4v. If you don't like the eddy carbs, you'll hate the qjets. They are more complicated that the eddies. Good luck.

I'm gunna go on a bit. What I found with the qjet is that everyone rants about the small primaries and the great mpg they produce. That is true, if you drive as if there is an egg between your foot and the pedal. Small primaries are great, but as soon as you get on it a bit, the booster opens to feed the engine, and there went your mpg.

Check your throttle shaft for play. Worn shafts/housings are common, and make tuning fun. You can bush the shaft.



Here is another problem area on the qjets. Dirt catches in the base of these idle tubes, and few remove and replace when they rebuild. Then when the carb doesn't run right they give it bad names.



If you look closely at the photo below, you can see a small restriction in the main air bleed in the top of the carb, the air horn. That is one of the many things Rochester changed to tune the qjet to specific engines. Those can be drilled to lean out the mix, or can be replaced with restrictions with smaller orifices to richen the mix.



have fun!

 

Kevin, you da man on Qjets! Always and all ways great write-ups on your projects.

 

It's never taken me a month rebuild a carburetor before i took this one apart and it sat on the workbench till about a week ago ... Anyways, my procrastination aside, i got the carb on the truck and couldn't be happier with the performance i've gained from it! I still need to fix a couple things on the engine before i can actually tune it, but from the start it was great! much better than an Edelbroke in my opinion.