Rochester Quadrajets tend to mystify people because of the various model names assigned to them by GM.
Add to that a redesign in the '70s and the confusion only grows.
In this video we'll learn the different model names and also how to differentiate between early and later-model Qjets. Enjoy!
How to de-bog your Quadrajet part 2
The Rochester Quadrajet has gotten a bad rap over the years, but it's
actually an excellent carburetor once you understand how they work.
Often referred to as "Quadrabog" or "Quadrajunk", many of the issues these
units suffer stem from neglect, or "modifications" that do more harm than
Being a spreadbore, air-valve secondary design, the Quadrajet is capable
of flowing 750 or 800 cfm of airflow, eliminating the need to buy different
carburetors when your set-up changes. In addition, the small primaries make
for great drivability and fuel economy.
In this clip, I'll discuss theory of operation on the secondary side,
since that's where most problems arise. So, leave your preconceived notions
at the door and let's see what makes these carburetors tick.
'77 Chevy K20: Quadrajet Choke Pull Off and Fast Idle Cam Observations
1977 Chevy K20. 350 crate engine. Rochester Quadrajet 4MV carburetor with
Truck suffers from unexplained hesitation and bogging when in forward
gears. Tends to rev just fine after a few minutes of mixed idling and high
revving. Checking choke operations as well as a possible sticky fast idle
cam as part of the process of elimination.
- Truck is allowed to idle/warm up until 2:25 mark, at which point the
engine is revved to about 2,000 rpm, and can be heard bogging (a hissing
sound can be heard). The fast idle cam is seen dropping. Fuel flow can be
seen in the glass-cased fuel filter right before the carb as remaining
- The engine is revved again about three more time up until 3:00 minute
mark. The engine revs slightly higher, but the hissing and bogging can
still be heard.
- Engine is revved again at the 3:35 mark, slightly past the 3,000 rpm
mark. Hesitation and hissing still evident, but less so than before.
- Engine revved at the 4:25 mark to about 4,000 rpm. No hesitation or
- Transmission is slowly shift through every gear starting at 4:55 mark.
Shifted back into Park at 5:05 mark.
- Engine revved again at 5:10 and 5:17 marks to 4,000 rpm. No hesitation or
hissing is heard.
To read up on the discussion, please visit the following thread:
750 vs. 800cfm Rochester Quadrajets-how to tell them apart
There exists a ton of misinformation regarding the actual airflow capacity
of the Rochester Quadrajet. Not many people know how many variations there
are or how to tell them apart.
Here is a simple video explaining how to identify each size based on
casting design. I will cover the "cfm-limited" carbs in a separate video.
1984 Chevy K20 50 to 70 mph
Punch it! I love to ear those secondaries open up on a four barrel carb.
This truck reminds me of the sounds used in a lot of 80's tv shows and
movies with car chases. I love it!
How to identify 4 barrel carburetors
Here's a guide to help identify the most common 4bbl carbs including
Edelbrock, Quadrajet, Holley and Thermoquad.
There are a few older or more obsolete carbs not shown including Rochester
4G, Autolite-Motorcraft 4100/4300/4350 and Carter WCFB that I'll detail in
the future provided I can actually find one.
How to measure and adjust Quadrajet float level
I have had several requests for this, so here you go:
1. The airhorn has to be removed to do this, but the carburetor does not
have to be removed from the engine.
2. Some data says to measure from the toe(end that is opposite from the
hinge), some say 5/16 inward from the end, etc. I suggest following the
specs for your particular carburetor.
3. With the float in the raised position and the inlet needle lightly
seated, measure from the top of the float to the top edge of the carburetor
with NO gasket in place. If you have to adjust the float, bend at the area
that has the cutouts,
4, If you are experiencing flooding issues then it is an excellent time to
install a new needle/seat and float and that should cure flooding problems
that are not choke-related.