Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

idle bypass question

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wannabemachinist:
 Hello I am new to this site and I am looking for some help. I have a 350sbc bored .030 over with flat top pistons, camel hump heads with a comp cams XE 262 with 262 deg intake and 270 deg exhaust duration. My qjet is a 1708 series 1984 year. The carb has the throttle shafts re bushed and a new kit in it. Driving it was fine but it could not idle properly with out it running fast in park. Covering the primaries with my hand made the RPMs increase and idle smoothly but no control with the idle screws. Reading Cliffs book I made some changes which helped a lot. I enlarged the idle tubes, idle down restriction, idle screw holes and the idle bypass. This is what I have now.
 jets are             72
 rods are            43x
 idle down res     .052
 idle tubes are     .040 were .032
 idle screws holes .092
 idle by pass         .075
 main air bleed      .075 lower and .120 in horn and .120 in bowel ( 2 orfice per side)
 initial timing         10 deg
 Idle is pretty good now and covering primaries with hand now drops RPM very slightly after a few moments but will not kill it. Still no control with idle screw though at least I am not seeing it
 If I remove a small vacuum line, idle improves some more and vacuum is guage is showing 15 which seems the best so far and I think there is response with the idle screws although very slight

so my questions are    do I increase the idle air bleed and how much?
                                  Am I still a bit lean?
                                   what would I change to richen it up if lean? jet and rods?
Thanks for any help you can offer 
     

Cliff Ruggles:
I'm confused with a few of your settings.  What is the rest of the carburetor part number and is it a single main airbleed model.

Most will be in and after 1980 unless it was a CCC unit or Marine unit......Cliff

wannabemachinist:
The complete carb number is 17084285 and I would say it is not a single air bleed. There is a .120 orfice in the air horn on each side of the screws and a .120 orfice just below in the bowl by the booster ring and the lowers are .075. I am saying there are 2 upper main air bleed per throttle bore at .120 each

wannabemachinist:
 Forgot to mention that my idle speed is about 700 to 725 rpm now in park and drops only slightly when is put in gear. This is when all unnecessary vacuum lines are capped 

Cliff Ruggles:
Most likely a Service Replacement for the late 1970's Chevy units with the large MAB's.

72 main jets are really lean in one of those and they didn't use 43X metering rods in them.  Are they "K" series instead?

I'm also unclear as to what the .075" main bleed is if the two other MAB's are .120".  Are you referring to the lower IAB?...

.040" idle tubes are huge but will certainly put some fuel to the mixture screws if that is what is needed to make the engine happy with that cam in it.

That is a tiny cam for a 350 engine build but the tight LSA still increases fuel requirements at idle speed.

Don't know what the quench is but flat top pistons and early Chevy double hump heads will yield pretty close to 10 to 1 even if the pistons were down in the holes some and thick head gasket used.  I build our 350 SBC's here targeting .025-.035" quench and they reward the end user very nicely with great power and manage pump fuel w/o issues even at higher compression ratios.

I've built scores of SBC engines dating clear back to the 1970's, and in the past 25 years or so have dyno'd most of them.  That cam does NOT make the grade or even come close.  The stock 350/300 hp cam had more duration than the Comp 262XE does.  The short seat timing and tight LSA make for a nice sounding engine at idle, but they don't make strong upper mid-range and top end power compared to longer duration grinds and/or wider LSA.

The best cam I know of (flat tappet) for an engine like yours is the CS-179R from Speed Pro.  It's basically a modern blueprinted version of the 327/350hp cam.  It requires 10 to 1 compression to work well, but nothing I've dyno'd against it makes more power over a broader rpm range, and it really shines if you add 1.6 rockers to it and Rhoads V-Max lifters lashed at .020".........FWIW.....Cliff

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