Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Idle mixture changing.

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CJK440:
Cliff got me up and running with parts to get my Edelbrock Qjet dialed in. Street manners are great. However I noticed my idle mixture fluctuates significantly from time to time. I have a wideband in the car. I'm not talking about the typical digital gauge fluctuation like from 14:1-15. I will have it adjusted to steady low 14's hot and maybe a few miles later at a light I will be at a light and it will be 16-17. I'm not suspecting the gauge because when its that lean I will hand choke it and it will rev up.

When the carb was rebuilt I replaced the idle tubes with new ones after I gave it a full cleaning. I tore it back down and sprayed it out to see if it was misc dirt and no change.

Any thoughts? I did notice in this hot weather that I drilled my idle air bypass too big. I cant lower idle below 1000 rpm, the screw comes off the lever. Could that be contributing? Is there an ideal number of turns of the curb idle screw on a Q jet to allow the best primary throttle plate angle?

77cruiser:
Adjust the idle to best vac. maybe a 1/4 turn lean & let the AFR be what it is. AFR at idle is iffy at best due to pulsation in the ex.
I suspect it's is going into misfire & that gives you a false lean indication as the sensor is reading oxygen, any misfire lets more oxygen out the ex.

Cliff Ruggles:
I'd also check the huge secondary throttle plates to make sure they are fully seating at idle.  Also make sure the linkage isn't holding them open slightly.

You'd have to add a LOT of bypass air to see 1000rpm's at idle speed, how big did you drill the holes to?....Cliff

CJK440:
Bypass holes are .110

CJK440:

--- Quote from: 77cruiser on May 25, 2018, 09:41:19 AM ---Adjust the idle to best vac. maybe a 1/4 turn lean & let the AFR be what it is. AFR at idle is iffy at best due to pulsation in the ex.
I suspect it's is going into misfire & that gives you a false lean indication as the sensor is reading oxygen, any misfire lets more oxygen out the ex.

--- End quote ---

When its idling lean and I richen it back to 14 or so(again using the wideband but also seeing how the idle responds to a little pump shot)  I'll go for a ride an find it really rich. So there is something intermittant I need to figure out.

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