Rochester Quadrajet carburetor 750-800 CFM explained

[@alanwylie8960]

I'm a retired certified GM and FMC mechanic, I think I've worked on every type of American built car/truck gasoline engine that was produced, the Rochester Quadrajet 750/800 CFM 4-bbl carburetors for the GM V8's is hands down the very best carburetor for stock to moderate modified engines up to 500-600hp. I had my fair share of hotrods growing up, like most hotrodders, I tried the various sizes of Holley & Edelbrock carbs and everytime, I'd immediately yank them off and put my Quadrajet back on. Also, I'm 100 percent convinced by experience that most people didn't know how to properly rebuild or tune a Quadrajet 4-bbl carburetor, including a lot of the mechanics that worked in dealerships or independent garages. I'm in the process of building a 350ci 375-450hp engine for my 88 GMC 4X4 truck, and it too will be powered with a Quadrajet 800CFM 4-bbl carburetor.

[@davidpotter7484]

Chevy trucks in the 80' had the 800. My 85 c20 is 17080213, i have 6 or 7 of them for spares and parts, same with my 72 c20 chevy 7042208. I have many dealer manuals, tune up books and 1985 gm rochester new product book that lists all the numbers. I have been told, by someone that wrote a book that my gm dealer books are wrong, and that was only made in 78-79. Lol, holy cow. 800 cfm carbs are all over the place on gm trucks still driving around.

[@porkrind3512]

I had a 71 GS buick 455 in the 80s. I was a big quadrajet fan. I didn't know about the bump. I never thought I had the 800cfm models either. Though my secondaries all went to 90°. On the 455, when you floored it, the vacum never went to zero. Which (as I understood it) meant it needed a bugger carb. I didn't want to go to a holley. I liked the sound of the secondaries opening up.

[@GlennLittleford]

I picked up a 800cfm model for my mild 350sb. A bit of tuning and modification, and its a perfect carb. No bog or flat spots, great economy, and 400hp on the dyno with perfect AF across the range. I'll never use a Holley again.

[@JosephCowen-fz8vj]

Yes i always wondered why the 403 Oldsmobile Small Block got a 800 cfm and the 400 Pontiac got the 750 , but looking it up it was all for emissions , the 403 s massive bore needed quick filling and responded better to a bigger bore carbie . As stated in GM litreture.

[@glennmanchester5696]

Pontiacs used the 800s on the 400 in the trans am for starters and im sure they used them in a lot of the big cu inch motors

[@thisisyourcaptainspeaking2259]

I really like both the quadrajet and the thermoquad.

[@blainesnider147]

Pontiac had 800 on RA4 and SD 455 for sure

[@PaulHayman-tq5kb]

Here in Australia is very hard to get the details On how to do it Wright and get to Run Wright when they they're going good just like fuel injection

[@geezman1000]

I have a milk crate filled with quadrajet carbs. I also have a milk crate full of holleys.

[@russellfinkler9092]

(I have a 7043250 ra 1223) qjet every place I look at all these videos and see they are all 750cfm or 800cfm. But called holley today for a replacement holley double pumper and they said a 600cfm is recommended for my olds 455 engine. Don't want to rob my car of hp if it needs. Let me know your thoughts thanks

[@sneakycobra5939]

Thank you! I couldn't find shit on this carb, I got this one my 75 cadillac 500

[@russelhough8609]

3 things that can ruin your effort rebuilding a quadrajet. Well plugs leaking, secondary air horn out of adjustment, and the sping under the primary needles stretched out by the previous rebuild.

[@jorgeadolfotrejogarcia2235]

I have a Qjet 750 just like the one you showed. When I increase the throttle gently the engine tries to shut down but if I close the choke plate with my hand the engine stays on. its acceleration. If I accelerate quickly to full throttle it does so but then it tries to shut down, as if it won't suck gasoline.

[@radperformanceswepco2372]

Seems to be a Vacuum leak

[@geezman1000]

I always throw a shop rag over the air horn. If it dies it has no vac leak if it doesn't then it's getting air some other way

[@vinskeeter]

Thanks for the video. Can the throttle plate bushings be replaced? (If they have them?) My idle is erratic, and I had to install a stronger return spring.

[@radperformanceswepco2372]

Yes you can! you will have to drill out center bore and tap in bushings. Some online sources sell kits for the Rochesters.

[@glennmanchester5696]

Mr heavy Chevy has a great video on q jets

[@glennmanchester5696]

I've always run nothing but q jets they are the best all around street carb ever made and the beauty of them is the air doors only allow the air the engine calls for in if the engine only calls for 550 cfm for example that's all it will get that's why they got away w only two models 750 and 800 and used them on everything small v8s to big blocks and all in between Pontiacs used the 800 a lot on the 400 trans am motors and many applications I'd have them moaning and my buddies always heard me coming for miles away lol best milage and throttle response due to the small primaries and when you open up the large secondaries it's a kick in the ass or it should be you don't get that w any square bore at all my opinion is hands down the very very best carb ever made for street use they last forever w out any adjustments set it and forget it no jet changes or power valves going out and spilling gas all over the intake manifold like Holley's Edelbrock aka Carter AFB which stood for aluminum four barrel then was bought out by Edelbrock is the second best and easiest to tune and make a decent street carb but a square bore none the less milage is not near as good and no kick in the ass performance out of them

[@PaulHayman-tq5kb]

Holden 253or 4,2 litre Blue motor Rochester has. Been nakered by a so called expert

[@glennmanchester5696]

Pontiacs used the 800s on the 400 in the trans am for starters and im sure they used them in a lot of the big cu inch motors

[@JosephCowen-fz8vj]

It was the 403 Olds in the Trans Am that got the 800 to pass emissions , the 400 Pontiacs got a 750 .