Is this carb OK?

[JohnnyPopper]

Where do you find such a meter Shifty?

[Shifty]

It's the $5 word for air/fuel ratio gauge....

https://www.autometer.com/2-air-fuel-ratio-elec-carbon-fiber.html

[JohnnyPopper]

THAT is very cool! Thanks!

At first glance, it seems like it could be a stand alone device. Or perhaps integrated into a electronic control system.

I need to do a deeper dive, but would love to monitor my AFR.

My truck has no silicon chips.

Just spoke to pre-sales, with their sensor and gauge it is stand alone, just need to weld a receiving nut to my exhaust pipe pre-cat. Great site Shifty one!!!

[Shifty]

Cheers Johnny!

[Chevygold]

Installed AEM AFR meter earlier, O2 sensor in LH header outlet so am able to monitor my AFR as I drive.
Now running 72 in the primaries and 75 in the secondaries seems a lot happier, haven't had a chance for a road test as spent a good deal of time helping my neighbour get his digger going again after running the fuel tank dry, absolute nightmare after the fuel line from the tank blocked up! lots of backflushing to the tank and finally got it going then had to take the boss shopping!
Graham

[Shifty]

How close are you able to get to that perfect 14.7:1 Graham?

[Chevygold]

Getting closer just a bit more juggling between the primaries and secondaries, just when it looks close the secondaries chime in and it all goes to sh1t but we'll get there, I won't be beat!
Graham

[JohnnyPopper]

Just a thought, but why go bigger on the secondaries if they're the same bore?

Seems you want the same AFR from both circuits, no?

[Chevygold]

Hi Johnny, the standard set up is 3 bigger in jet size in the secondaries than the primaries so guess it's more about flow than just the size of the chokes?
Another thing that always puzzles me is generally your vacuum decreases the more you stomp on the load pedal (I'm old enough to remember vacuum wipers! the faster you went the slower the wipers went so you had to slow down to see where you're going) so how does the vacuum open the secondaries when you go faster if it's dropping when you floor it?
Been taking advantage of the hot weather we're having now to get on with a bit of paint, got the side of the bed stripped and primed ready for colour, man that Chevy paint is tough stripper didn't touch it ended up with a flapper disc in the angle grinder!
Graham

[JohnnyPopper]

LOL my '57 Apache Chevy pickup had the same wipers!

I guess they want a real dump of fuel when the secondaries open rather than double the volume in total.

I think the vacuum keeps the secondaries closed until it drops...spring loaded I suppose.

[bd]

I guess they want a real dump of fuel when the secondaries open rather than double the volume in total.

I think the vacuum keeps the secondaries closed until it drops...spring loaded I suppose.

Think about it for a moment...  Is there a choke plate above the secondary throttle plates?  Generally, when would you use the secondaries?  When would you want them to open?

If engine vacuum keeps the secondaries closed and spring pressure opens them, wouldn't the secondaries be fully open when the engine is shut off?  Are they open with the engine shut off?

Ponder it.  What do you suppose?   

[JohnnyPopper]

I have no idea how square bore carbs work, never owned one so I can only guess.

His comment about the secondaries opening when the vac drops lead to my speculation.

What's the dope bd?

[bd]

JP, speaking generalizations while kicking back around a cracker barrel, look at it from a "lay" perspective.  A carburetor is a carburetor.  They all operate using the same principles of pressure differentials.  With the exception of variable venturi carburetors such as the Quadrajet or AVS or VV, secondary bores typically are unobstructed by choke plates or air valves.  So even if the primaries and secondaries are square, meaning the same size (not to be confused with square bore, which means the bore spacing is equilateral), the secondaries will flow more air than the primaries.  Besides, secondaries typically open only when there is a high load demand such as rapid acceleration or when the vehicle is pulling heavy loads.  Hence, secondary fuel metering is enrichened to provide maximum power production from the engine.  So your first guess was correct.

As to the vacuum secondary operation, the secondary throttle plates are held shut by spring pressure pushing through the secondary vacuum diaphragm and its link.  The secondary diaphragm is ported to a variable vacuum signal that is initially dependent on the airflow past the primary throttle plates.  The greater the airflow through the primary bores, the stronger the vacuum signal.  Albeit, some carburetor designs augment the primary bore vacuum signal by blending it with a secondary bore vacuum signal for smoother secondary throttle opening and transition.  The sensitivity of the secondary throttle opening is effectively controlled by the pressure balance established between the diaphragm spring and the ported vacuum signal.  Tuning the secondary throttle opening is accomplished by installing the proper spring for the desired result, opening sooner or later.  A well-tuned vacuum secondary carburetor will provide a seamless transition from engine idle to full power production.

Not a perfect explanation, but you get the idea.

[JohnnyPopper]

Wow, I am a smarter man today! Sincere thanks bd!

Sounds like Graham needs to look at the spring tension of his carb secondaries to effect the proper balance.

I have only ever worked with Qjets for the last 40 years. You list them with variable venturis and I am not making the connection.

They have a triple venturi that I know of.

The secondary (not chokes) upper plates, to my understanding, respond to the flow the secondary plates create, lifting the metering rods to allow more fuel.

Is that what you are referring to?

[bd]

QJ secondaries are mechanically tied to the primary throttle opening through a spring-loaded lever mechanism.  The spring loading allows the secondaries to be locked out by the choke until the engine has warmed sufficiently to safely generate high power without imparting engine damage or causing undesirable lean backfires and acceleration bogs.   

As you stated, the secondary air valve opening depends on downward airflow through the secondary bores.  And, yes, as the air valve opens it lifts the secondary metering rods to increase fuel delivery through the secondary nozzles.  The datum you are missing is that the air valve's position or angle of opening effectively modulates the cross-section of the secondary bores, and with that, the venturi effect at the secondary nozzle openings.  Consequently, the purpose and function of the secondary metering rods are not directly comparable to the purpose and function of the primary metering rods.  Rather, the secondary rods compensate for dynamic changes in the effective cross-section and airflow potential through the secondary bores due to the air valve angle, whereas the primary power piston and metering rods enrichen the fuel mixture delivered through the fixed cross-section of the primary bores based on increased engine load.  Do you see that?  The secondary metering rods compensate for airflow; the primary metering rods compensate for engine load (power demand).  Essentially, the secondary metering rods, following an initial dip, will maintain a flatter AFR response to changes in secondary airflow, while the primary metering rods will decrease the AFR (increase fuel delivery) in response to increased power demand. 

Another way to express the QJ concept is that the primaries comprise a fixed-bore two-barrel carburetor and the secondaries comprise a variable-bore two-barrel carburetor.  It's the variable venturi attributes of the QJ that made it so adaptable to the entire GM lineup.  Truly, it was an engineering marvel.