Hy Guys, i have a original 7041540 quadrajet on my 455 electra 71. The quadrajet is overhauled and runs ok, when the engine is warm. But my mechanic told me at the last check that the lambda value is very low, so the mixture is too lean, especially at higher rpm. (Value 0,39 - should be 0,9). He said that the main jets should be changed (0.73), but he has no experience on which value. What value should I start testing with, 0.77? Or should i first try to change the second jet needles. They are original AS. The engine is completely overhauled with a slightly larger bore and planned heads, and runs with a standard camshaft very smooth. Thanks for help!
It is pretty normal for Quadrajets to run on the lean side especially under light throttle and cruise conditions. If you are concerned that the engine is too lean at wide open throttle under high load, you can increase the jet size to increase the WOT fuel. If you go up 3 or more primary jet sizes, go up 1 primary rod size. So if you go up to 76 jets, go to 45B rods. You can use thinner secondary rods to tune the secondary side. For a completely stock engine, I wouldn't move away from the stock calibrations. Lamda and AFR readings can be misleading. Does the engine hesitate, surge or run badly? You might be trying to fix something that isn't broken. JMO.
Thank you for sharing your knowledge! The car runs very well. Very rarely I notice that it hesitate a little bit at kickdown and low rpm, but that is not important. I will leave everything as it is, and sometime check the cup seal from the accelerator pump. The pump is on the inner hole.
Look at the primary vacuum break. Make sure it isn't inoperative. The biggest reason a QJ will bog or hesitate is because the secondary air valves (up top) flop in too quickly. The primary vacuum break has a dual purpose. It opens the choke blade after the engine starts on the choke, AND it slows the air valve opening upon primary to secondary throttle transitions. There is also a spring wind up adjustment for the air valves. With the engine OFF, you should be able to tip in the air blades with a finger, and they should close smoothly and quickly when you remove your finger. With the engine RUNNING at idle, the primary vacuum break should hold the air valves closed.
Thanks again! The vacuum break is ok, but i havn't seen the adjustment of the air valves spring until now! I check this.
If Lambda is < 1.0, that is rich, > 1.0, that is lean. If you are 0.39 Lambda, that is very rich, what condition was this measured? Your carb should have 43B rods and 73 jets, are you sure you don't have an internal issue? Did you calibrate your sensor in free air? Not many combinations need a 77 jet.
Thank you also for your replay. In the meantime, I have found out myself, that the value I wrote above can't be right. Unfortunately, I don't have the measuring device myself, and it's been too long again that I can still assign the values correctly now. Possibly it was the idle gas value that I remembered, or any other value. What is certain is that my mechanic meant, that the values are too lean at higher rpm. And the rods an the jets are original like you wrote. But at the moment i can't drive and wait around 5 weeks for the new Radiator, because the old one (with new net) is brocken (other side). I hope before Winter break i can test the values again by my mechanic, and write them down better. Until this i rember the sentence from Larry: "You might be trying to fix something that isn't broken" But another question: What influence does the change of the secondary jet needles then actually? There are also different sizes.
The secondary jets are all .1360" and non removable from the carburetor. The secondary rods have different thicknesses along their entire length. As the secondary air valves rotate open there is a plastic cam that lifts the secondary hanger and rods. As the rods get lifted out of the jet, the air fuel changes varying with the amount of jet uncovered. Here is a chart of secondary Quadrajet rods. The rods are identified by a 2 letter code. Buy this book for more information, https://www.amazon.com/Rebuild-Modify-Rochester-Quadrajet-Carburetors/dp/1932494189
