Author Topic: Lean idle  (Read 959 times)

Offline 53Olds

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Lean idle
« on: September 15, 2018, 03:55:57 PM »
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.

Offline 53Olds

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Re: Lean idle
« Reply #1 on: September 15, 2018, 04:40:47 PM »
Should have included the carb number: 17059250

Offline Cliff Ruggles

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Re: Lean idle
« Reply #2 on: September 16, 2018, 03:24:44 AM »
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

Offline 53Olds

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Re: Lean idle
« Reply #3 on: September 16, 2018, 08:48:42 AM »
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.

Offline 53Olds

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Re: Lean idle
« Reply #4 on: September 22, 2018, 10:53:58 AM »
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.

Offline Sonny

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Re: Lean idle
« Reply #5 on: September 22, 2018, 11:23:22 AM »
DCR confounded me too at first. In the book it is noted as Idle Channel Restrictor.

On my Quadrajet the DCR looks like a brass cup pressed into the idle down channel. The cup has a small hole drilled in the bottom. Your logic for opening the restriction to fatten the idle makes sense to me.

Please take my newbie status into account when considering my reply.  ;D

Offline 53Olds

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Re: Lean idle
« Reply #6 on: September 22, 2018, 11:39:39 AM »
Sonny, that clears up the "What is a DCR?" issue perfectly.

I can fit a .039" drill bit into the hole with wiggle room, but a 1/16" (.0625") bit is too large to fit, so I'm not sure I want to just enlarge it without more precise measurements. Recipe One calls for .046", Recipe Two calls for .052-.055", Recipe Three calls for .059-.064".

I'll wait a bit for Cliff to weigh in. I'd hate to open it too far.  :-[

Offline Sonny

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Re: Lean idle
« Reply #7 on: September 22, 2018, 12:09:04 PM »
Yes wait for input from Cliff!!!!

Also consider checking for obstruction in the orifice at the bottom of the idle tube. Mine had gunk there and after a cleaning the measurement was .006 larger


Offline Cliff Ruggles

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Re: Lean idle
« Reply #8 on: September 23, 2018, 04:09:50 AM »
Not sure why you would drill the holes under the mixture screws to .125", that is HUGE and the metric mixture screws may no longer seat.  I've built (last time I looked) over 12,900 carburetors since 2003 and have yet to take the holes under the mixture screws past .110", and I keep most down around .090-.096".

Sounds like you need a set of numbered drill bits vs trying to gauge hole sizes without them.

This is exact science, so be precise when drilling items out for improvements.

With a .038" idle tube in that carb you should have PLENTY of idle fuel to the mixture screws with .055-.059" DCR's.  This assumes the upper and lower IAB's are still the stock sizes. 

I would NOT drill any air bleeds out in that unit, idle or main, they should remain stock to work correctly with the parts I sent.......Cliff

Offline 53Olds

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Re: Lean idle
« Reply #9 on: September 23, 2018, 10:46:02 AM »
Good news, when actually flipping the carb over to see the idle screw passage while inserting a drill bit, only a 5/64" drill bit extends all the way through. When checking earlier, I was just going by feel, and the larger bit seemed to move on in. My mistake, it doesn't, in fact even a 3/32" bit does not pass through. So, without precise bits in that range, we can at least say the idle screw passage is between .078" and .0938", which seems to fit the recommended range. It might even need to be enlarged further? The idle screw does seat fully when turned in all the way.

I picked up some wire bits last nights, but not a complete set as the only store in that town only sells them individually, it's 75 miles away, and I'd rather get something with a case to keep the bits organized. My eyesight isn't what it used to be, even assuming those tiny bits have sizes stamped on them. I could probably find a more complete set in Billings, but that's 120 miles, and I really hate spending $50 on gas to get a $30 set of drill bits. Mail order would take a week. At any rate, the DCRs are larger than .039"  and smaller than .0520". Should I drill them out with a 54 (.055"), 53 (.0595"), or something else?

I haven't done anything to the idle air bleeds, so they are presumably still stock.

Offline 53Olds

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Re: Lean idle
« Reply #10 on: September 23, 2018, 02:34:53 PM »
Drilled the idle mixture screws passages out to .090", the DCR holes to .0595".

Seems to have solved the lean problems.  :)

It now idles nicely at 650-700 rpm, good idle mixture control with the mixture screws (now at three turns), no sign of leanness.

I've changed the vacuum to the distributor to ported vacuum, rerouted a couple vacuum lines, and bumped the initial timing up to 8 degrees. So far, so good.

Offline Cliff Ruggles

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Re: Lean idle
« Reply #11 on: September 24, 2018, 03:16:40 AM »
Good news.  .059" is fine for DCR's on that carb.  I would not go any larger with the holes under the mixture screws......Cliff

Timothytuh

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Lean idle
« Reply #12 on: January 29, 2019, 05:25:26 AM »
Sounds like the primary to me too. If boost is not lean then you are getting fuel. Could be the FPR too? Maybe its letting too much by at idle? Do you have any way to reading fuel pressure?

Do you have the dual stage injectors or are both the same ccs?