Idle mixture changing.

[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]

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.

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.

[Cliff Ruggles]

Correct the high idle problem first.  I suspect you may be pulling some fuel off the boosters (nozzle drip) at that rpm.

To idle that high it's getting too much air from someplace, or too much timing at idle speed.

What is the engine combo?

What is the distributor doing and how is it set-up?....

[CJK440]

Fixed the idle problem. When I pulled the carb to clean it thinking maybe dirt was causing the idle fluctuation I installed a new base gasket and didn't torque the front bolt enough. Now if I back the idle screw out all the way it drops to 500 rpm hot.

Is there an ideal primary plate position for idle? I know the rule of thumb with holleys was to have the transfer slot exposed as much as it is wide so the part of the slot uncovered looked square.

Any similar rule of thumb for the Qjet.

[Cliff Ruggles]

Not really, but the throttle angle must be low enough so three things don't happen.

First you can't be uncovering any ported vacuum supply slots or holes.

Second, there can not be any fuel coming from the boosters (nozzle drip).

Last, the plates can't be so far closed that they are sticking in the bores.  It really helps to index and align the plates and install bronze bushings for the primary throttle shaft to help things out there as well......Cliff