That's right folks, I installed a rebuilt Rochester M4ME electric choke Quadrajet carburetor on my '76 300! I went with the Offy DP intake, and an Edelbrock-brand spreadbore-to-squarebore adapter, flipped upside down (it was intended to put squarebore carbs on GM intakes....SURPRISE!)

Also done at this same time was the swap from the log exhaust to EFI headers and the Walker y-pipe and a Dynomax 2.5" cat-back. (cat not installed yet)

Don't hate me folks. Chrysler used spreadbore carbs too. Ford is the odd duck out, and shame on Ford for using the tiny YFA. I'm glad they let the six breathe better with the wolly-mammoth spare ribs EFI setup. This is a Rochester carburetor. Not a Delco. It was manufactured for GM; GM had the good sense to use this great carburetor, and in great numbers for nearly 30 years. I got a very good remanufactured carb, with all the tricks and mods done to it; epoxy on the leaky parts, new brass bushings on the primary shaft, the whole nine yards.

After 12 gruelling hours of pounding, grinding, sawzalling and "there's no way this is going to work", the motor ROARED to life like the flick of a switch. No sputtering, no hesitation. Just VROOM! Some tweaking, tuning, adjusting and a coat hanger later, it was idling contentedly in my friends driveway and the right pedal ran the butterflies. I really owe my friend for his great help in fabricating a throttle bracket and donating a fuel inlet fitting off his project Z28. See? GM owners aren't all bad... :-)

There's a couple of exhaust leaks and ticks up front, but it's liveable for now, and will be addressed in subsequent tuning sessions.

How 750CFM drives on a 300CI motor:

It drives fine. It drives great, even. It's far more sensitive at low RPM than the Carter YFA it replaced (fewer vacuum leaks?), and it has some GRUNT. Some of that is also due to the new exhaust. Every time I pulled away from a light, I was stunned at how the truck surged forward, in any gear. It just went. No stumbling. No hesitation, and this is out-of-the-box "generic V8" tuning. Gas mileage wasn't bad either. Maybe a little less than I was getting with the YFA, but some new jets and time with a wideband O2 oughta fix that. The primary bores add up to 165CFM, I think, and the secondary bores are vacuum-regulated, so there's never more bore than the engine can handle. On the highway, it was still limited by the stock cam. No getting around that (until I install my Crane 260 dual pattern. :-)). But the truck jumped to attention when I gave it more gas at speed. The old YFA just made more noise at the exhaust and sort of sped up. Again, 30 year old carb with vacuum leaks, but still.

I'll post more details later. Right now I'm beat tired and am going to shower and collapse into bed.

 

Rochester (fuel systems) and Delco (mostly electonic parts) are both divisions of the General.

 

Oops. My mistake. Can I plead ignorance on this one? :-)

I chose the Q-Jet based on a bunch of opinions in 4x4 forums and a purportedly successful Slant Six installation. The forums said Holleys never ran right, and AFB variants would starve and die climbing a speed bump. 2BBLs like the 2150 were good, but limited due to being a single-stage design. They loved the Q-Jet because of (quoting) a central fuel bowl, tiny primaries, and you can change the jets just by pulling the air filter lid.

Slant six references:
http://cars.rasoenterprises.com/Carb-Quadrajet.htm
http://cars.rasoenterprises.com/Carburetion.htm

 

Ford used spread bore carbs also, just not on very many applications. I believe they were all Holley's.

I've had several Holley carbs and never had a problem getting them to run right.

 

All right, all right. Enough schooling me. Can I get some congrats for upgrading my truck?

 

That's one of the things girlfriends are for.

By the way, Q-jets have a basic flaw apparantly you haven't heard before....there are two plugs at the bottom of the bowl. Over time, the sealant cracks letting fuel leak into the manifold slowly. If you haven't driven in like three days, there's no fuel in the bowl, you crank it, doesn't start until the fuel pump has delivered new fuel and filled the bowl...elapsed time...about 10 seconds.

 

Those plugs were replaced and coated in fresh epoxy by the remanufacturer. And at least one other place that these carbs are known to leak. I did a lot of research on rebuilding a q-jet myself before I finally selected a reman unit, and I have a lifetime warranty on it.

 

Hey, I'll help you out. The Rochester Quadrajet is, IMO, the best carburetor ever devised by man. As carbs go, it's the next best thing to EFI. The primary metering circuit is very sophisticated and they are extremely reliable. If you stay out of the secondaries, they give great MPG and driveability (owing to the aforementioned primary metering). I am a Ford snob, but I love the Q-jet.

The main well plug leakage is pretty well known, hell I was repairing those with special GM (gas-resistant) epoxy in the early 80s. Hardly new news.

Sounds like a great mod, BB. Enjoy!!

 

im not a big fan of the quadrajunk. ford used a spread bore autolite 4300 series 675cfm or something close to that on the late model 460's

 

CONGRATULATIONS!!!

I always liked those carbs, those mini-primaries worked well. They will suck the old wallet dry if you drive in the secondaries tho.

 

I put some of the photos I took in my gallery. It's a pretty clean looking install, and will only get better when I reroute the heater and A/C hoses. Makes the rest of the engine bay look really dingy.

I'm just amazed at how simple it all is. I hooked up...fuel, throttle and 12V to the choke. That's it, and the engine runs. Nothing was complicated. I didn't miss some critical step. Getting all the right fittings and bolts together beforehand was stressful, because I just didn't know what was going to be neccesary. I didn't know if I was going to break a manifold bolt or have scarring on the head mating surface from the old leaky non-existent gasket. The only things I wasn't sure about were the linkage and fuel feed, and those got taken care of with a coathanger and a donor car.

 

The little carter that couldn't.
I run the EFI setup, but latley I've been having problems with the engine running rich....enough to where I get about 3 miles out of 5 gallons of fuel and a set of plugs......If anyone knows about the EFI versions of the 6 cylinder 300, please please let me know. Any help is appreciated. Anyway, to "Big Blue" Sweet,,,interesting, but Sweeet

 

check your fuel pressure regulator. most likely its bad and putting way too much fuel to the engine. too check you need a gauge you can hook up to the valve on the fuel rail.

 

faststang94- Nah, it's not running rich- the plugs seem OK and the exhaust doesn't stink at all. I don't think the carbureted 76 I'm talking about in this thread even has a fuel pressure regulator.

 

no i wasn't refering to you, i was refering to the guy i qouted that posted before me saying his efi 300 was running really rich.