Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Edel 1901, AMS out, Still Lean at Idle

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bry593:
It will be a few days before I'm back on the carb.  I've got to get the dash back together.  Went in to fix the clock and blower switch, found some bad connectors and wiring. 

bry593:
I am back on the carb and have drilled Recipe 1. 

First, I might say that it appears the Edelbrock carb is about as lean as possible.  The holes are much smaller in diameter than what Cliff recommends for a mild engine.  However, the body main air bleed is very large at .120".  I wonder if Edelbrock changes the size of the main airbed as a means to tune the entire system?

I haven't reassembled the carb at this point since I managed to lose my power piston spring, and I have a question for Cliff.  I had already drilled the airhorn main air bleeds before noticing the body bleeds were larger than Recipe 1.  What negative effects should I expect from a total main bleed of .07 + .12 over .07 + .07?  Should I try to plug the .120 and re-drill?  This is a large motor (413 CI) with a small cam, so maybe it's not an issue?

As I mentioned, I applied Recipe 1 and had to use both 0-60 and a 60-X drill sets.  I inserted drills into the holes until there was a no-go condition, and then assumed the previous size drill bit as the diameter.  A dial caliper was also used to record measurements before and after the holes were enlarged.  For the following data, the before diameters are listed first; followed by after and equivalent drill size:


* Idle Mixture Holes: .077 ; .087 #43
* Idle Bypass: .098 ; .098 #39
* Idle Tube: .029 ; .036 #64
* Idle Down Channel/Restrictors: .046 ; .046 #56
* Lower Idle Air Bleed: .062 ; .070 #50
* Upper Idle Air Bleed (in body): .070 ; .070 #50
* Accelerator Pump Discharge: .026 ; .026 #71
* Main Air Bleed (in body): .120 ; .120 #31
* Main Air Bleed (in air horn): .050 ; .070 #50
* Primary Jet: .069 ; .071 #71
* Primary Metering Rod: .035 3EE; .045 45B
* Fuel Inlet Seat: .110 ; .125
* Float Level: .420 ; .313
* Secondary POE Well Restriction (in body): .032 ; .038 #62
* Secondary POE Discharge (at flap): .052 ; .052 #55
* Secondary Tube Restriction (innermost two): .026 ; .036 #64
* Secondary Hanger: K ; K
* Secondary Metering Rod: DR; DA
* Air Flap Open Distance: 1.295; 1.295
* Secondary/Air Flap Spring: 7/8 ; 3/4 turns
* Choke Pull-off Release Time: 3s; 3s

bry593:
I talked to Cliff and he said the .120" main air bleed in the body is going to cause issues with this large motor and small cam.  It will simply pull too much air.  I've ordered some brass plugs/restrictors and will install these to reduce body bleed to .050 and also change the airhorn bleed back to what it was originally, .050".  It was suggested that I return the lower idle air bleed (just above the throttle body plates) back to the original .050" as well.  The #71 primary jets might be replaced with #69, depending on how rich everything seems after the orifices are resized.  The APT screw is going to be made functional so I can adjust the power piston height.  As a final recommendation, the vac advance is being routed to ported vacuum.  This will allow me to open the throttle blades a bit at idle, making the idle circuit function better.

Hope to have the modifications made by end of next week.  With any luck, the 1970 Caprice will be on a roadtrip the following Saturday!   

bry593:
Pulled the apt screw.  Was loc-tited in so had to heat the base with a torch for about 5 secs.  Came out easily while still hot.  Before removing the screw, I measured spring pin to base as a reference, was .118".  Will set it back to this height and later adjust to where it is just slightly lean around 2000 rpm using the hand over the venturis method.

Hope to see my parts by Wed.  Anxious to have the old Chevy going again.

bry593:
I've made the recommended changes:


* Main air bleed (in body):  .050
* Main air bleed (in air horn):  .050
* Primary jet:  #69
* Idle bypass:  plugged
* Choke pull off rate:  2s
To change the choke pull off rate from 3.5s to 2s, the orifice was first drilled with the smallest bit in the set (.0135?).  This changed the rate to 1s, too fast.  I then silver soldered then port closed (metal housing) and then ground one side until I could just see a crack between the solder and the port.  This resulted in a 2s rate.

Main air bleed setscrew purchased from Cliff Ruggles:


Idle bypass plug, made from 1/4-20 UNC X 3/4 aluminum, pan head screw.  Purchased at ACE hardware, sectioned into two pieces, slotted with a Dremel and secured with red Loc-Tite.


Tuning the carburetor, I found max vacuum to be 16 in-Hg with idle screws backed out 3.5 turns.  Backing these out farther had only a slight effect on idle, however the vacuum began to drop.

Idle speed can now be controlled with the idle screw and it was set to 550 rpm in drive (about 650 in park).

I bought a new APT screw and spring from Cliff.  It is much longer than the original, so much easier to access with a screwdriver.  The screw was adjusted at 2000 rpm, and richened until a slight tilt of the choke flap caused no increase in rpm.  It was then leaned out until the tilt caused approximately a 100 rpm increase.  This was found to be 1/8 turn richer from the original setting.

A test drive revealed a noticeable improvement in primary circuit operation.  However, there is a slight hiccup (hesitation) when going into the secondaries.  Cliff, what would you recommend to address the secondary stumble?

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