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Old 07-26-2013, 10:01 PM
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Idle Air Bleeds size and the transition circuit

Hello,

I need some help understanding the idle air bleeds and the way their size affects the transition circuit.

I know the basics where they control the air/fuel mixture going to the idle port but I want to understand the effect the bleeds and the size used have on the transition circuit.

As I read it, not only will a larger bleed lean out the mixture but it will also delay the effect of that circuit. I would like to understand exactly what that delay means and its relation to the transition and main circuit.

When I purchased my carb the engine shop said I needed 74 idle air bleeds (up from the stock 70) based on my engine/cam combo. I am now trying to understand why that is; I asked but was told that my cam will need it to idle properly. (it does idle fine but doesn't explain it at the level that helps me understand the why)

I have experimented by putting in 66's and after adjusting the mixture screws I really did not see a difference but I am likely missing something so anyone who can shed light on this would be appreciated.

PS. I read tons of articles on this and I have not yet found one to clearly explain the effect of the idle bleeds once you get past idle on the throttle scale.

Thank You for any assistance!
Gary

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Old 07-26-2013, 11:21 PM
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There is no way in hell that shop could know the exact required idle air bleed size.

If you want to dial in the off idle transition circuit get a AFR gauge-meter.
This will allow you to see the off idle afr ratio. and see the effect on that by tweeking the IAB's
by hand while ti is running, by partially blocking them with your fingers. This will shift the off idle AFR richer
as you block more flow thru the IAB's.
It is all about gettign a balance between the IFR size and the idle air bleed.

Delaying the circuit has to do with the HSAB's effect on the start up of the main jet circuit
as the throttle opens ,, not the IAB on the idle circuit.

These are two different things. Do not assume the IAB needs tweek just cause someone told you so.
If the car does not shift lean as you come off idle and onto the idle transition ( Tslot) then the IAB size if fine)

Wheh adjusting the idle mixture be sure the throttles are correctly positioned at idle, first.
Then set up the idle mix screws using amanifold vacuum gauge for best idle quality, not a particular AFR. at idle.
Then while watching the afr gague, open the throttle a bit and note the AFR. If it goes too far lean, the IFR is too small or the IAB is too big. If it shift way rich then the IAB is too small or the IFR is too big.
Very small changes +.001" -.001"make big difference, so any changes must be precise. Like hand honing a IAB using a drill bit just a bit at a time by hand and recehcking.

You can use a Wideband or a simpler cheaper heated narrow band AFR gauge for this.

Controling the amount of air flow thru the PCV system at idle also effectively trimmes the whole idle circuit.

If the PCV valve is wide open ( high flow pindle position) at idle (wrong PCV valve) it will have a off idle flat /lean spot too. so the PCV valve matters and can really throw it all off.

Why do you think the IAB's need tweeking anyway? just off idle driving stumble? ...or ?

idle timing and idle throttle T slot position setting at idle is critical. PCV valve flow rate at idle.
Get these all correct first ,,before playing with the IAB's or IFR's. as these rarely actually need tweeking at all.


It all has to work together. what carb? what cam? what idle issue?
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Old 07-26-2013, 11:31 PM
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When I do have to tweek the IAB size, I don;t run out and by a bunch of air bleeds.
Just get a pair of spares and solder them up and redrill with a .062" drill bit and then hone with the drill bit, to exact size, by hand,
to get the required finished size. Your need a afr guage for this. and the exhaust system must not be leaking.

tweeking the PCV valve flow rate is also very very effective. You cannot correct the wrong PCV valve choice by tweeking the IAB's

Again , a shop may advise that the IAB's may or may not need adjustment but there is now way in hell they will know the exact required new IAB size.

if the carbs's metering blocks came off of some other size carb then you can get real lost. real fast.
"Frankenstein carb"

Last edited by F-BIRD'88; 07-26-2013 at 11:38 PM.
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Old 07-27-2013, 12:31 AM
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Idle Bleeds

Hey F-Bird`88,

That's what I thought, the company is, in the end, trying to make money so I knew I would have to learn and adjust as required.

Let me answer your comments one at a time:
* I do have a Daytona wego AF hooked up so I am definitely using that in my tuning.
* I have the quickfuel bleed kit (65-75) so need to drill.
* Thank You for the explanation on the `delay`, now that I know it affects the HSAB that helps a lot.
* The shifting off of idle is fine with either the 74`s or the 66`s; the afr gauge shows in the mid 13`s either way; I will double check though as now I know what to look for.
- one question, when do you know your shifting off of idle onto transition? the way I was shown was to determine the RPM at which point the mains start dribbling which is 1600 in my case.
* Throttle plates/transition slot is set to 0.037 (0.020 secondary) and I don`t touch that since set. (it may need to be but I this is a good starting point and sofar has not caused me any issues)
* I used a vacuum gauge to set to maximum vacuum (14 in my case); I ignore AFR but I like to reference it afterwards; although I did receive an interesting comment that a high overlap cam will cause AFR to read rich at idle. Since I`m using manual methods to set idle and using AFR only as a reference I don't think this is an issue.
* when opening the throttle a slight bit I do notice a high AFR (18-19) but what do you mean with `slight`, I have the problem at 1400 rpm (from 880 idle) and I can duplicate it by putting the carb in high idle which is approx. 1400. I managed to somewhat get rid of it by shifting fuel from the secondary to the primary bowl (ensuring balance was maintained) but I know its not properly resolved.

The whole PCV valve size is a discussion I have not gotten anywhere on. I have found a number online for a high flow unit (if that`s even correct) so any information on how to fix that are appreciated.

I have been able to dial in my IAB`s no matter what the size but I want to understand what they do to the detail level so I can understand all the tuning details. Consider it the OCD side of me

Carb is Quickfuel ss-680-vs and no idle issues, just learning (althought for all I know I do have issues but don`t know it yet so hence to learning/question period).

Cheers

Ps. as to your next post;
* exhaust is not leaking
* 100% agree that a shop will not know what I need but I do listen to their opinion and then use it in conscious decision making.

Last edited by ggevaert; 07-27-2013 at 12:52 AM.
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Old 07-27-2013, 11:01 AM
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cam

forgot to include cam specs; they are: 450 lift, 224 duration @ 50.
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Old 07-27-2013, 11:24 AM
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* when opening the throttle a slight bit I do notice a high AFR (18-19) but what do you mean with `slight`, I have the problem at 1400 rpm (from 880 idle) and I can duplicate it by putting the carb in high idle which is approx. 1400. I managed to somewhat get rid of it by shifting fuel from the secondary to the primary bowl (ensuring balance was maintained) but I know its not properly resolved.

Then you should be able to swing the afr back richer by using your fingers over the primary IAB's, while watching the AFR gauge. Primary IAB's just a hair too big.. ( WRONG PCV VALVE ^ very likely*)
IFR a hair too small... More than likely it is the PCV valve calibration mismatch.

readjust the primary and sec T slot exposure after adding more idle base timing.
The primary throttle position (T slow exposure) may be too far open at idle.
More idle timing allows readjustment.

106LSA cams need a lot of idle timing.
The trouble with PCV valves is there is no reference chart for different applcations or flow specifications
and pindle position vacuum points.
to allow you to choose the correct one for your engine and cam.

Thank your/the USA's EPA for that.

GM does sell a simple "fixed orifice" type PCV valve that you can modify for flow rate as required, if you want.

Note: main circuit "delay" is a function of the Main HSAB air bleed, main emulsion well hole calibration and poistion, main booster venturri design and position and on some carbs the "kill bleed", not just the HSAB alone.
It all works together. Messsing with the HSAB can really screw up the top end AFR shift and burn up a motor, so don;t get all carried away trying to adjust the main start up point "delay". one effects the other. overall balance is the key.
again very small changes in the HSAB size has a big effect. 98% 99% of the time it needs no adjustment. ever

Getting the right PCV valve is a critical critical factor here. I lucked out and got one from Canadian Tire that work great .
Lots of different ones to choose from but no specifications to go by.

You can manipulate your PCV valve (internal pindle valve) with a small screwdriver and see the effect at idle and offidle while watching the vacuum gauge and AFR gauge. You can restrict the PCV hose with pliers too and see the effect on the AFR at idle/off idle/transition.

Last edited by F-BIRD'88; 07-27-2013 at 11:42 AM.
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Old 07-27-2013, 11:45 AM
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when all set up at idle, how may turns out are the idle mixture screws?
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Old 07-30-2013, 07:04 PM
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Afr

Hi,

I'm out of town this week but will work on this weekend.

F-Bird'88, I was out at 60% of a turn on all 4 corners.
Also, I did what you mention about the finger over the IAB by inserting a 0.017 welder wire (all I had on hand) and the AFR dropped from 18 to 15 at 1400 RPM.

On the PCV valve, based on your info my thought now is to run the tuning without the PCV valve and close off that orifice on the carb; then when the carb is dialed in I will hook various PCV valve's up and watch the response through idle/off idle on the AFR to find one that works.
If you have the part # for the GM one I would appreciate it.
Also, I should also mention that, at the dragstrip, I had some oil coming out of the dipstick tube so I have high internal pressure; I was already working on looking at running a open air filter instead of a PCV valve just for the strip as I was thinking it is the culprit. (the other side has an open filter already)

I have been told by a number of folks to leave the HSAB's alone unless you have the experince and fuel mapping technologies so I'm not touching them at all. When I told quickfuel my issue they told me to go down 2 sizes on my HSAB's and I bought them but I will not be putting them in unless I can get confirmation they are the only answer.

On sunday I went back and pulled the carburator out and reset everything back to the base setting. While i kept careful notes before I am sure something got changed without being recorded so I want to start with a known base and go from there.

So here is my starting point:
* both primary and secondary transition slots are set to 0.020". I used a 0.020 feeler gauge to verify. Interestingly enough when I held the Carb up to the light I noted that the secondaries show significantly more light around the butterflies around that setting so that is something I need to keep in mind.
* IAB's reset to 70's (from 74) - which is how the carb comes from the factory.
* verified that accelerator pump is set to 0.015 and has a 31 shot in it. Left the brown cam in though.
* left PV in at 7.5 (was originally 6.5)
* PVCR's were never changed and are 0.049
* IFR's were never changed and are 0.031

And finally, I just found out an engine builder company has a dyno literally a mile from where I live so I will be going there to see if they rent it out.

Thank You
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Old 07-30-2013, 09:03 PM
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what is the new afr when at 1400 rpm +/- with the carb now returned to baseline and the .070" primary IAB's?

Take two of the Now spare HSAB's and use then as custom primary IAB's and get a .062" drill bit. (1/16" common) try that. Then if it needs to be larger than .062" hand hone a bit at a time with the drill and retry. Reset the idle mix screws correctly each time.

probabily end up about .067". You are very close already. a off idle Transition AFR of 13's to 15's is good.
smooth drivability is the priority.

Final check should be done with the AIR CLEANER INSTALLED...

Just curious... What is the name of the engine shop?

GM fixed orifice style PCV valve #12572717

Another you can try ... Canadian Tire # 17-9283 ( chevy Chevette PCV valve) {FRAM FV190}

Last edited by F-BIRD'88; 07-30-2013 at 09:16 PM.
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Old 07-30-2013, 09:18 PM
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Cool PCV valve hack and some PCV valve info
Why you should use a PCV catch can (pics)

Valve cover baffle design soda pop can
http://i287.photobucket.com/albums/l...anbaffle4a.jpg

Combined opposing side valve cover breather with dirt filter
http://i287.photobucket.com/albums/l...baffling6a.jpg
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Old 07-31-2013, 04:55 PM
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Carb tuning

Thanks F-Bird'88,

Will get those PCV's in.

The shop I was dealing with is 4 seconds flat (just search that in google) in oregon. I purchased a carb/fuel system/intake combo from them as I knew my original setup was inadequate for the engine I had and they had good pricing.

I have the quickfuel 64-90 range of IAB bleeds (bought the kit) so I can change them as needed. Will try all of this when back in town.

Cheers!

Last edited by ggevaert; 07-31-2013 at 05:01 PM.
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Old 07-31-2013, 05:11 PM
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Valve Cover breather

On the valve cover breather designs; nice but I seem to be a magnet for the Ministry of Environment so I want to ensure that I don't vent to the atmosphere. ($380 per item they find, so with 2 valve cover breathers that would be too much )

Currently I have this setup (see included pic) where it goes into the bottom of the air cleaner. The PCV setup goes into the carb in back of course.

Cheers
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Old 07-31-2013, 05:21 PM
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Pcv

So, I just read the PCV article; Am I understanding it properly that you don't want large amounts of air to travel from the breather to the PCV? I was actually trying the opposite and have as big of an opening as possible in the hope it gets rid of my crank case pressure issue. My next thought was that the PCV valve is now a restriction point, hence oil coming out of the dip stick tube so I was looking at ways to increase its size/throughput.

So I should actually be restricting the amount of air going into the breather side? Less air in the system means less pressure then?

I'm thinking I should do a leakdown test to eliminate other potential issues.
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Old 08-01-2013, 06:50 PM
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The PCV valve itself is not the cause of the dip stick tube leaking oil.
This is not a indication of excessive blow by or poor crank case vent.
it means oil is getting to the top of the dipstick tube.

is the breather huffing oil @ WOT? is oil getting up into the PCV hose or in the intake manifold?

Are the spark plugs showing oil contamination? is the engine down on power?

But getting the right PCV valve for your engine with a non stock camshaft is critical.

there is nothing stopping you from building a PCV/vent system that is Smog fuzz friendly "closed type" yet does the job.

In a court of law, there is a big difference between tampering disabling and optimizing a custom "closed" PCV system.
The burden of proof will be on you.

if you drive your hot rod car in a jurisdiction that has active enforcement, yes you need to be on your game.

Last edited by F-BIRD'88; 08-01-2013 at 07:00 PM.
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Old 08-03-2013, 02:11 PM
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Carb tuning

Ok, had a chance to do some tuning; here are the results.

* All this was done without a PCV with the PCV hose plugged going into the carb.
* Below is the best RPM/HG I could get for each scenario

With Constant Vacuum
* In Park
* Timing is at 31 degrees
* Rpm = 1100
* HG = 14.8
* AFR = 13.5
* A/F mix screws are at 85% of one turn out

With ported Vacuum
* In Park
* Timing is at 21 degrees
* RPM = 970
* HG 13
* AFR = 13.5
* A/F mix screws are at 85% of one turn out, I could not get any improvement to RPM/HG by adjusting them further than what was already set with constant vacuum.

~~~~~~~~~~~~~~~~~~
Continuing with Constant Vacuum settings

In park (with air filter on - 3" tall 14" unit)
@ 1400 RPM = 14.2-14.6
@ 1650 RPM = 15.2
@ 2000 RPM = 17.0
@ 2100 RPM = 18.0
@ 2200 RPM = 14.3
~~~~~~~~~~~~~~~~~
In drive (with air filter on - 3" tall 14" unit)
Idles @ 900 RPM
AFR = 14.5
HG = 12

In Drive and
@ 1250 RPM
AFR = 16.2
HG = 12.5
@ 1350 RPM
AFR 18
HG = 11.6

I also tried putting in the GM adjustable PCV valve with stock opening; here are the results while idling in Park.
* Constant vacuum
* RPM = 1090
* HG = 14
* AFR = 13.8
* Did not try any AF mixture screw adjustments.

Gary
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