Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

quadrajet 7042210 idle question

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gclark:
With my engine idling at 800 rpm, if I pull the cap off of the vacuum port on the side of the carburetor, the engine speed increases by about 75 rpm.

Does this mean that my engine wants more bypass air? 

Is opening the vacuum port a valid diagnostic for the idle circuit?

Thanks in advance for your help.



My engine and carburetor specs:
355 SBC, 10.4:1 CR   
TFS 23 Degree Aluminum Heads   
Hyd Roller Cam: 110 LSA; Timing @ 0.050: 214 Intake, 220 Exhaust   
1.5:1 Rocker Arm Ratio   
MSD HEI Ignition   
TH350 Automatic Transmission   
1972 Chevy C-10 Pickup   
   
Quadrajet 7042210   
Idle Tube   0.036
Idle Channel   0.047
Lower Idle Air Bleed   0.070
Upper Idle Air Bleed(airhorn)   0.052
Accel Pump Discharge Holes   0.024
Main Air Bleed(main body)   0.070
Main Air Bleed(air horn)   0.070
Main Jet   0.074
Primary Metering Rod   43B
Fuel Inlet Seat   0.135
Float Level   1/4"
Bypass Air     0.084

omaha:
It is possible yes or maybe the idle is just a tad bit rich. Of course if you try to lean out the idle and the rpm's drop off then yes, I would think the carb might need more bypass air. It depends if you can open up the primary butterflys without any nozzle drip, this is a hard thing to tell as far as I'm concerened. (when it's just on the edge of needing bypass air) I guess adding a bit of bypass air would not hurt because it is easily plugged.  I guess bypass air is better than the butterflies in any case but go small and "creep up on it".  Getting an engine to idle correctly can be a real PITA sometimes, there's the timing, the throttle opening, vacuum reading, bypass air and A/F ratio. I remember the carb in my old VW was bypass air only. The throttle was totally closed during idle. {PICT 34 Solex carb}. Just my .02.

Cliff Ruggles:
Did you move the upper idle airbleeds to the airhorn? 

Were the upper idle airbleeds blocked off in the main casting?

Did you install the main airbleeds as well?

Were you able to free up the APT screw in the baseplate?.....Cliff

gclark:
All- Thanks for the responses.

Cliff- yes to all of the above.  I rebuilt this carburetor after discussions with you on how to set up the airbleeds and free up the APT screw.  I used your kit with the replacement APT screw. 

The engine smells a little rich at idle.  Just wondering if the test with the vacuum hose means the engine wants more air/less fuel at idle.

Thanks,
Geoff Clark

Cliff Ruggles:
Do you have control of the idle fuel with the mixtures screws?

A lean setting will stink as much as, if not more than a rich setting.

Often with tight LSA (110 or narrower) cams, a rich idle mixture is required to get a smooth idle, or decent idle.  I don't like or use tight LSA cams for most street engines for this reason.  They sound great, lots of "attitude" etc, but allow a lot of unburnt fuel out the tailpipes due to the additional overlap and poorer scavenging at idle speed(s)......Cliff

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