Idle Air Bleeds size and the transition circuit
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!
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).
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 01:52 AM.
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.
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.
Last edited by ggevaert; 07-31-2013 at 06:01 PM.
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.
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.
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
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.
Carb Tuning con't
Did some more testing with closing down the IAB's and here is what I found:
Hopefully this chart comes across ok.
Long story short; significantly (by half) decreasing the IAB's (by 0.035) gets rid of the high AFR at 1400 RPM (in drive).
0.017 wire in IAB's
In secondaries only; 13.1 @ 1050 (idle); 16.7 @ 2000
In Primary and secondaries; 12.8 @ 1050 (idle); 15.6 @ 2150
In secondaries only; 12.8 @ 930 (idle); 16.7 @ 1400
In Primary and secondaries 12.8 @ 930 (idle); 16 @ 1430
In secondaries only; 12.4 @ 1080 (idle); 15.2 @ 2150
In Primary and secondaries; 12.4 @ 1050 (idle); 13.5 @ 2300
In secondaries only; 12.4 @ 890 (idle); 15.6 @ 1440
In Primary and secondaries; 12.3 @ 890 (idle); 12.8 @ 1330 and 14 @ 1500
Must have been typing my last post when you replied.
I noted now that, in Drive it has 13.5 @ 930 so not sure if the 14.5 was a wrong reading or my writing it down wrong. Either way, that is where it seems to have settled on.
I will switch it over to Ported and redo some of the testing; specifically, the 2000 RPM in park and in drive (which turns out to be 1400) as well as inserting the 0.035 wire in park and in drive to see what the AFR's are.
I still have no hooked up the PCV; My thought is to get everything working without it and then, afterwards, introduce it into the mix and adjust it a bit at a time. Trying to limit my variables here.
I did read a ton on porting vs non-porting and I have to say there are lots of discussions explaining their case both ways
To answer your other questions:
1. I did not touch the primary or secondary idle speed screws at all so the transition slot in both will still be at 0.020. (with the second butterflies showing more light at that setting than the first).
2. the GM PCV valve is just a shaped open space; there is nothing in it. It is straight (no 90 degree plastic piece on the end), all metal, with a very small opening at the end that goes into the valve cover; I can see the term adjustable in this case means you can drill out the hole bigger
3. Haven't taken the car out yet (couldn't get out of driveway as we had the driveway full of contractors working on our house)
So, based on all this info; what does this mean. I'm trying to digest it all.
I'm back to where I was before; I had a smooth idle and good driveability but I had that high AFR spot at around 1400 (but it moves around depending on how I change my front to rear throttle balance but never quite goes away).
Thanks for the continued assistance.
Carb Tuning continued
Finally had a full day where I could test/research and made some good progress in both understanding and Carb tuning.
First a question:
* If your idle mixture screws are only out 1/2 turn from close does that mean the entire idle circuit is too rich and needs to be leaned out? (i.e. smaller IFR's)
As noted previously I reset the entire Carb back to factor default leaving only the PV at 7.5 (my lowest HG during various normal, very little load, city/hwy driving conditions is 9.5, I idle at 14 hg). Here are the settings on my Carb now - I am leaving main jets out as I'm only concentrating on idle/transition circuit at this point.
- Using ported vacuum at all times.
- 70 outer idle air bleeds
- 28 high speed air bleed
- Accelerator Nozzle – 31, orange cam
- no secondary accelerator
- Power valve – 7.5
- PVCR size - .049 each
- Idle feed restrictor – 0.031 each
Got the vehicle idling smoothly at 970 RPM in park (880 in drive) with an AFR of 13.8. increasing the idle to 2000 in idle (1500 in drive) continued to caus a very lean spot (18-19 AFR) between 1600 and 1900 (note that mains start to come in at 1950).
I then did the IAB to IFR balance test
- by turning the mixture screw in 1/2 turn and then out 1/2 turn and seeing results of both on the RPM gauge; In my case the idle speed increased when I enriched it so my IFR was lean so I increased from 0.031 to 0.033.
I then inserted welders wire (0.030) in the IAB's and that corrected the high AFR reading down to the mid 13's so I dropped the IAB size to 55 (lowest I had) and then went for drives around town and on highway checking AFR at various points (30 mph, 45 mph, 60 mph). I made sure never to increase the throttle too agressively to either get the secondary or PV involved.
Result is that my lean spot is gone but I am noting that I am often dipping into the low 13's on the AFR so I would like to correct that. I am thinking the entire idle/transition circuit is to rich now in most situations.
my thoughts are as follows; any suggestions are very much appreciated:
* Decrease the IFR significantly first to allow me to open up the air mixture screws to around 1.5 turns out. Then readjust mixture screws to get back to an AFR of 13.8. (Not sure how much to decrease IFR so still working on that)
* Work on the IAB's to get the off idle and transition to have an AFR around low 14's and aim to have the transition circuit drop off at 1950 rpm where the mains come in. I am thinking I will need a rather large IAB to compensate for the low IFR size.
Finally, the accelerator. I have been ignoring it (other than ensuring it is set to act instantly and have 0.015 left at WOT) in my testing but I know it adds fuel to the mix so I need to keep it in mind. Do you run the carb off-idle/transition generally lean and then use the accelerator to fill in the additional fuel to bring it to an AFR of 14's? That would then assume that it is ok to have a lean environment of 15-16's when cruising through the city as your not using the throttle a lot then - correct?
Mains coming online at different times
Another item I noticed; is it a problem when the boosters start coming online at different times? I noted the left booster started dribbling at 1950 rpm and the right one at 2100 rpm.
|Recent Engine posts with photos|
|Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)|
|Thread||Thread Starter||Forum||Replies||Last Post|
|idle air bleeds||1957willyman||Engine||3||05-03-2013 01:04 PM|
|Transition circuit on Holley 4160 too lean.||johnt365||Engine||0||02-17-2013 08:57 AM|
|carburetor question about idle and air bleeds.||eric32||Engine||4||03-20-2012 07:11 AM|
|BG/BC Rich Idle & Idle Air Bleeds/IFR||68NovaSS||Engine||3||11-11-2006 12:36 AM|
|Air bleeds Holley HP (idle & main)||Bumpstick||Engine||0||07-14-2006 08:38 PM|