Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Big cam, stinky rich idle

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dave at hsp:
Howdy! I'm new here so forgive my mistakes.
I talked to Cliff about this a few days ago and he gave me some info of what to do, so this is sort of an update. First some back ground. This is an '80 Camaro, 305 automatic, 350 turbo with a 2500 stall converter, 3.73 gears. The cam is too big for this application, I know. But, it's in there and that's what I'm gonna make work.
 I rebuilt the engine. It's .030 over with flat top pistons, so the compression is now 10.2 to 1. Stock heads. New Comp Cams Thumper cam, lifters, springs, retainers, locks, timing set, installed at 102° intake CL. Its .479 and .465 lift and 227 and 241 @ .050. The carb so far is as follows:
 idle tube: .042
 Idle down channel restriction, brass thread in : .063
 Lower idle air bleed, brass thread in: .052
 upper idle air bleed: .070
 Accelerator pump discharge holes: .028
Main air bleeds ( base plate) .140
Main air bleed (air horn) .120
 I have the lightest spring in the power valve that comes in the Edelbrock 4 spring kit.
 It will idle warm at about 1000 in park, pull it in gear and it drops to between 500 and 800 with a big chop, which is OK, but the idle is stinky rich. Wing the throttle and it rev's right up. No hesitation or stumble. The vacuum at idle in park bounces a bit between 5 and 8 inches. In gear it's barely 5 inches.
 My question is, where do I go next to back the idle screw off about 1 turn to get the throttle plates to where I'm not exposing so much of the transition slot, and still maintain about 800 rpm at idle, not so stinky rich? How big is too big on the main air bleed in the base plate?
 Thanks for any comments, ideas, answers.

Dave

ejowrench:
You did not mention if you have idle bypass air, Are you familiar with this?  With a cam like that, you will most likely need it.

Are you experiencing nozzle drip at idle?

dave at hsp:
Yes, I am using idle bypass air. The main air bleed in the base plate is at .140. That is my main question; how big is too big on the main air bleed in the base plate? (for the idle bypass air)
I also didn't mention the timing. The timing is set at 14° initial, 12° vacuum advance, 16° centrifugal, all in at 3500 rpm. The vacuum advance is full vacuum, with a can that comes in at 3 inches and is all in at 8 inches of vacuum. That is part of the problem. Trying to carry enough vacuum to hold the vacuum advance at idle, so the idle won't drop below about 800 rpm. Once I loose the vacuum, I loose the rpm being held by the vacuum advance. If I turn up the main idle screw, the butterfly goes into the transition slot and pulls fuel from the booster.
 I don't see any drip from the booster at the current setting.
 I did check the AFR this afternoon with my LM-2 and it was 13.0 in park at 750 rpm and 13.5 to 13.9 in drive at 550 to 600 rpm.
That was fully warmed up, and it did have a slight, very slight stumble on acceleration. No problem with a quick wing of the throttle.

Thanks.
Dave

ejowrench:
Dave, I always like to plumb my vacuum advance to a ported vacuum source, and then set the initial timing to suit. I believe Cliff recommends this in his book. I'm not sure if this is your primary problem, but it is worth trying.

Marx3:
the idle afr sounds pretty good? like a computercontrolled car almost...
What about afr at cruise and wot?
on a sidenote: I have the same LM-2, great little instrument. How far up the tailpipe do you mount yours ? I homemade a bracket for mine, it goes about 2" up the tailpipe, so I often have trouble getting trustworthy readings in idle. ( 25:1 ). No problems once driving though.

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