Will a Q-Jet Idle Rich & "Load Up" if Power Piston is not Seated at Idle? I Test it..
 

Fact or Fiction: If the Q-Jet carb's Power Piston is not fully seated at idle (due to the piston spring being too stiff, or engine vacuum being too low), will the carb idle poorly, idle rich, and cause the engine to "load up" at idle? I hear this all the time: "I need a softer spring for my power piston, because the carb is running rich at idle - it's because the piston is not seated."

Is that really true...?

A while ago, I did a test on a Holley carb to see if the same myth about Holleys is true: "If the power valve is open at idle, the carb will flood out and run rich at idle." See my test results here:
https://www.corvetteforum.com/forums...gine-idle.html

An open Holley power valve has no effect on idle. So will a Q-Jet behave the same, and be unaffected by a power piston in the "full rich" (raised) position at idle?

Here's the test article: I was testing a 1977 Q-Jet for a Forum member this afternoon, and his carb is running really well after rebuild and setup. Here his carb is running on the test engine:
https://cimg8.ibsrv.net/gimg/www.cor...3ae609ae91.jpg

Idle speed is right around 850:
https://cimg0.ibsrv.net/gimg/www.cor...faa4e0567f.jpg

...and idle air/fuel mixture is right on the numbers at 14.7:1
https://cimg7.ibsrv.net/gimg/www.cor...846da832fa.jpg

Carb is off the test engine and partially disassembled to reveal the power piston. Here the power piston is in the full-rich, "raised" position. This puts the "skinny" tip of the primary metering rods in the jets, richening up the mixture when the car goes into wide open throttle:
https://cimg8.ibsrv.net/gimg/www.cor...70d00b8b8e.jpg

When engine vacuum is good, the piston pulls down against the spring, and the "fat" part of the rod goes into the jets, leaning out the mixture at cruise. On cars pulling good vacuum, the piston will be seated in this "lean" position at idle:
https://cimg7.ibsrv.net/gimg/www.cor...42cbe266d2.jpg

But if engine vacuum at idle is poor, it's possible that the power piston could be in the raised, "full rich" position with only the skinny power tips of the rods in the jets. Will this make the idle go rich and screw up idle quality..? Here you see the rods attached to the piston with the taper and different diameters near the tips of the rods:
https://cimg9.ibsrv.net/gimg/www.cor...39496ef3af.jpg

So to create the most adverse, rich condition possible, let's not just jamb the piston in the full-rich position... let's completely remove the rods from the carb so the carb is grossly rich. This should REALLY screw up the idle if the myth holds true... Here I have removed the rods from the piston rod hanger arms:
https://cimg9.ibsrv.net/gimg/www.cor...1783668a65.jpg

...and I have re-installed the piston into the carb bowl without the rods. I wanted to keep the piston in the carb rather than removing it completely since the power piston seals against manifold vacuum applied below the piston - if I removed the piston, I would have a vacuum leak:
https://cimg5.ibsrv.net/gimg/www.cor...9e43a4145b.jpg

The carb is back on the test engine, and it's up and running again. The engine sounds and feels remarkably "normal":
https://cimg2.ibsrv.net/gimg/www.cor...e355a06e66.jpg

I didn't touch the idle speed screw or the idle mixture screws. All I did was remove the rods. A quick check of the idle speed shows idle speed to be completely unchanged:
https://cimg2.ibsrv.net/gimg/www.cor...768d557dbb.jpg

...and the idle air/fuel mixture is EXACTLY as it was with the rods in the jets. There is absolutely no change at all:
https://cimg7.ibsrv.net/gimg/www.cor...a3d90298ee.jpg

The idle circuit in both the Holley and the Q-Jet is independent of the main metering circuit. The power enrichment of the Holley power valve and the Q-Jet power piston add additional fuel to the main metering circuit. The idle circuit on both carbs pulls its fuel from the main fuel well in the main metering circuit, but the amount of fuel it pulls is controlled entirely by the idle fuel restrictors, low speed air bleeds, and the idle mixture screws. Power enrichment circuit has no effect on idle.

So why would a big-cammed car be rich at idle, idle poorly, "load up", and have no response on the idle mixture screws? This is usually caused by engine tuning (mostly timing issues) that require the throttle opening on the carb to be so big that the carb is not actually idling on its idle circuit: The throttles are open far enough that fuel is discharging through the transition circuit and from the main discharge nozzles. To solve this problem, the throttle plates must be closed down. This can be achieved with more ignition timing at idle; secondary throttle idle adjustment (on Holley); or increased idle air bypass flow. But it won't be solved by using a softer power piston spring...

:thumbs:
Lars

Excellent info Lars.

So....Idling on the idle circuit. How does the transition to the main circuit happen? Obviously it begins with accelerator pump movement. If you were to lightly 'blip' the throttle without rods what would happen? How crazy rich would it go?

This kind of stuff should be added into a carb tuning sticky file above.....

Flicking the throttle and operating the accel pump off-idle in a no-load condition is no different with or without the rods - the main discharge circuit does not come into operation at light load. "Blipping" the throttle without rods (or with the power piston in the full rich position) does not result in a rich condition at all. Once the carb gets into the main metering circuit at cruise speed with some load (with some airflow through the venturies), the rich condition would become very apparent, and jabbing the accel pump would result in an over-rich condition that would cause a hesitation and black smoke out the exhaust.

The transition to the main metering circuit happens from idle through the transition circuit. The accel pump provides immediate fuel addition to compliment the immediate increase in airflow as the throttle is opened - due to the "mass" of the fuel, the fuel is slower to "respond" than the air when the throttle is opened, so a temporary and immediate fuel "shot' is needed whenever the throttle opening is increased. The transition circuit, operating in parallel with the idle circuit, provides an immediate fuel flow increase just above idle, and before there is enough airflow to "trigger" fuel discharge from the main discharge nozzles. Again, the power enrichment circuit (rods) has no effect on this transition circuit, so if you were to operate the vehicle at very small throttle openings (just off idle), you could actually drive the car without rods (or even primary metering jets) with no difference in engine performance. But as soon as fuel flow is triggered through the main metering circuit, things will go bad very quick...Don't try to drive your car without rods or jets... :thumbs:

But, Forrest, then everyone would know all my secrets...!

Lars

some Oldsmobile Q jets had no metering rods

That's correct. That was the 1970 455 W30. That carb had the smallest primary jets ever used in a Q-Jet due to the deleted rods: The jets were #58. This brought the main metering circuit metering area to roughly the same metering area as other Q-Jets with primary rods in their larger jets.

Lars

I wasn't aware of these. What was the rationale behind that design for the W30?

Live well,

SJW

The W30 455s were essentially factory built drag race cars. Deleting the power piston and rods is the same racing trick as removing and plugging the power valve on a Holley for racing - you then compensate for the lack of a power enrichment circuit through jetting. Doesn't do much for gas mileage, but it makes for an easily tune-able racecar. :thumbs:
Lars

if you are rich at idle and the idle mixture adjustment screws have no effect you could have the idle transfer slots exposed too much because the butterflies are open too far because of a hot cam

That's correct. Point of this thread is that this condition is usually blamed on the power piston spring, which is not the cause of the problem. See the last paragraph in my original post, which states that the problem is excessive throttle plate opening. :thumbs:

Lars