Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Lean idle

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53Olds:
Working on dialing in the carb.

Specs: Olds 425, vintage 1965, 10:1 compression. Erson cam, 110+4 LSA, .5355 lift (.504 at the cam plus 1.7 Harland Sharp rockers), 284/284 duration. Pertronix Ignitor III, Performer manifold, dual exhaust. 5 degrees initial timing, 40 degrees total, manifold vacuum to distributor.

Rebuilt carb according to Cliff's specs. Opened up the idle tubes, the idle ports, clipped a coil off the idle screws.

It runs well after break-in and tweaking, although it sounds like a sewing machine. Might need to adjust the rockers. They're quiet at idle, noisy going down the road - but I've never had full roller rockers before, so don't really know how noisy is "noisy".

Can't get the idle to smooth out perfectly, lean at all idle mixture settings. Screws are out seven turns. Can't find any vacuum leaks - tested all hoses, sprayed manifold, carb base, throttle shafts, etc. No more than a mild smoothing anywhere, roughly equal to covering the mouth of the carb. Idles now at 700 rpm, 17 inches of vacuum, but covering the mouth of the carb results in gaining an inch of vacuum and 100-200 rpm. I went through a number of combinations for timing, vacuum advance, vacuum hose positions, etc, and this seems to be the best combination - but the lean condition at idle was a constant condition. Covering the mouth of the carb to improve the mixture and then spraying for vacuum leaks basically didn't change idle, so I can't believe there are any significant leaks anywhere.

It seems like it just wants more fuel at idle, but I'm not sure what my best approach is.

53Olds:
Should have included the carb number: 17059250

Cliff Ruggles:
Exactly what did you do to the carb?

Idle tube size, DCR's, idle airbleeds, bypass air.

Jets, metering rods, power piston spring used.

Did you put one of our rebuild kits in it?.....Cliff

53Olds:
Left my notes at work, but yes, all parts were from a kit purchased 8/7/18, mods as per phone call. I can update tomorrow with specifics.

53Olds:
Due to over-zealous janitors, notes no longer exist.

Pulled it apart again, and here's what I've got:
Primary jets: 73, changed from 74
Primary rods: Unknown, no markings. I have a vague memory that you were sending 53P rods, but can't swear to it. Previous rods were 55P.
Spring: Orange/red
Float level: Neoprene, 13/32"
Secondary hanger: E
Secondary rods: DA
APT: 3 turns
Idle screw passages: Can't remember if I drilled them, but I can fit a 1/8" drill bit into them but not a 9/64", so about .125"
Idle tube: .038"
Bypass air: Drilled .110" in baseplate

Best overall tuning: 8 degrees initial timing, 39 degrees total, running manifold vacuum to distributor. Idle screws 7 1/2 turns out. Idles at about 700 rpm, a bit ragged. Fully warmed up, this gives about 17 inches of vacuum (using a new vacuum/fuel pressure gauge from CarQuest. Do I trust it? About as far as I can throw it, but...). When covering the mouth of the carb with my hand, rpm increases 100-200, vacuum increases to 18", idle smooths nicely.

I readjusted the rockers, eliminating some of the sewing machine noise but not affecting the lean condition or vacuum readings. I also replaced the newer PCV valve with the one that I was using before the rebuild. Since the original engine used a breather, not PCV, the metering is guesswork. The PCV valve I am using now actually did seem to improve the situation slightly - before, capping the PCV valve would result in a minor improvement in the idle, now capping it doesn't seem to affect the idle.

I'm guessing I need to open the DCR, because I actually don't know what that stands for. Down Channel Restrictor, maybe? From reading through many, many other posts, they seem to be in the main body next to the idle tubes, but I can't find a specific mention of them in your book, at least not by the acronym.

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