Radar,
I always get my N idle settings close and then fine tune in D with the brake applied.
I am currently idling at 1050rpms in N and it dropped to 850rpms in D. The only thing that I can think of is that I am idling on the transition circuit at the 1050rpms and when it drops to 850rpms in D, it is then on the idle circuit.
I am going to drop my N idle to 975 and see what the O2 readings are. If this is correct, the N O2 readings should lean out some more.

Thanks for the reply.

I got my first day out in the car with the O2 sensor. This is what I found (67 degrees):
N=1000rpms 13.4-13.6 15"hg (rich?)
D=800rpms 13.6 11"hg (rich)
light accel from stop = 13.8 (good?)
light accel with pv open = mid 13's (good?)
Cruise:
40mph= 2200rpms 15.8-16.0 (lean?)
45mph= 2400rpms 15.6 (lean?)
50mph= 2500rpms 15.4-15.6 (lean?)
55mph= 2700rpms 15.4-16.0 (lean?)

I did not test WOT. I was just out for a cruise for the first drive of the season.
Are the 15.4-16.0 readings too lean?

Is the 2200-2700rpms on the main jet circuit or am I still on the transition circuit? I believe that 2200rpms and up are my jetting but I am not sure.
I have read where 4 jets equal 1 full point on an O2 gauge. With my cruise reading high 15's, should I bump up my primary jets by 2 which might be equal to a half of a point on the O2 guage? This would put me in the low 15's if this works out right.

It was VERY windy (45mph + wind), but I did not notice any surging or pinging sounds.

Let me know what you think.

Thanks

Jim,
I will test to see what the O2 gauge shows on my next drive past 2700rpms.
Current setup:
Quick Fuel SS-750 Annular DP, 66/72 jets, 8.5 hi flow pv (per QF), .028 IFR's, .075 IAB's, 4 metering screws out one full turn. I do believe the buttrtflies are 1 11/16". I think that I am far enough into the throttle to be on the primaries.
Thanks Jim.

These are my updated numbers after another cruise in 76 degree weather.

D=800rpms 14.6 11"hg (this was 13.6 in 64 deg.)
For unknown reasons, the car ran leaner in the warmer 76 temps then it did earlier in 64 deg temps.

light accel from stop = 15-16 (was 13.8 in 64)
Same as above. Leaned out in warmer temps.

Cruise: (Transition lean issue?)
30mph= 1700rpms 17.0 (added)
40mph= 2200rpms 15.8-16.0
45mph= 2400rpms 15.6
50mph= 2500rpms 15.4-15.6
55mph= 2700rpms 15.4-16.0

CHECK THIS OUT
CRUISE: (main jet a tad rich?)
3000rpms in 2nd gear= 14.8-15.0
60mph= 3100rpms 13.6-13.8 (added)
3500rpms in 2nd gear= 13.6-13.8 (same as 60mph)

It appears that the mains are starting in between 2700rpms and 3000rpms. My main jets (66's) might be a tad rich. I swapped out my .071 IABS to the current .075's after the first startup with the O2 kit. Should I go back to the 071's to bring the mains in quicker and to richen the transition?

TRANSITION:
30mph= 1700rpms 17.0
Off idle with a red pump cam and a 037 squirter, I am still getting a lean spike of 16 on the guage from a stop to light throttle. I believe that the 071 IAB's will help this too but I am not sure if my .028 IFR's are too lean too looking at the 30mph thru 55mph readings.

Thank you very much.

The IABs have a greater effect with higher air flow. However, the bigger IAB may also delay transition circuit flow. I don't consider myself an expert by any measure, but my thought would be to decrease the IAB size some since it is an easy change, and see how it affects the off-idle transition and low RPM AFR. If that doesn't adequately improve it, then come up a bit on the IFR.

I've been tinkering with my QF Q850PV; it has been rich all over the range (admittedly it is sold as a race carb), but I'm slowly getting closer to improving its street manners.

Dave

YO7:

At this point, I'm most concerned with the occasional 16 and 17 you're seeing. Even 16, where you are not reporting any misfire issues, "might" be on the hairy edge of too lean. Some engines will tolerate mixtures that lean, some will not. Plus, you're measuring 4 cylinders. One cylinder might be leaner than the rest. Before you go any leaner with the MJs, I'd really like you to pull all 4 spark plugs from whichever side of the engine the sensor bung is on, just to be sure everything looks OK.

I agree wholeheartedly w/DaveNC regarding the IABs...go back to the 71s (or smaller), it's quick and easy. You can always pop in larger IFRs if smaller IABs aren't doing it for you.

You're really not far from where you want to be. It appears from your last post that you aren't getting leaner as low-load RPM increases, something I was concerned about.

Keep us posted.

J

I have .073, .071, and .067" IAB's. I will test the .071's (factory for this carb) and retest later on in the week.
I will shoot for my cruise numbers to be in the 14.5-15.5 range.

EDIT:
Notes:
- I spoke with QF this morning and they suggested that I try and have my main circuit active between 2,000rpms-3,000rpms as a general rule. So far I am right at the high end of that range (2,900rpms-3,000rpm). The smaller air bleeds should get me down lower into that range.

- I have read online where a general rule of thumb on changing jets due to the AF ratio is 4 jet sizes equals 1.0 point of AF change. I asked QF about this and they said that due to the different signals of all of the different engine combos, there is no general rule of thumb when trying to change jets. This is still a test and tune deal. Currently my mains are showing 13.6/13.8 so this puts me about 1 full point lean. I am just trying to think ahead but I need to finish my transition tuning first. I have plenty of jets to choose from but right now, I am guessing at a 2-3 jet size drop on the primaries.


Thank you very much for the replies.

Did the QF rep provide any more color to the statement about starting the main circuit between 2K and 3K? I'm still learning the fine points of carb tuning as well, but I don't believe the IABs will have an effect on the main circuit start.

On my car, it appears that due to limited throttle opening, the transition circuit continues to deliver a good bit of fuel, even up in the 3K range (highway cruise), where the mains are starting. The combined fuel leads to a pretty rich mixture. I have been debating on whether I should try and slow down the main activation.

Thanks for your thoughts!

Dave

Dave,
I found this explanation on the web.

"Adjustable air bleeds, while usually found on high-dollar fuel mixers, can have a significant impact on A/F ratio. Not only will larger air bleeds lean out the mixture, they will also delay the effect of that circuit by weakening its signal.

Conversely, installing smaller (richer) air bleeds boosts the signal and speeds up the transition into that circuit. These factors have great influence over driveability."

In my case, since the mains are not starting until apx. 3000rpms, and I need to richen the transition circuit, by installing smaller idle air bleeds will do both. They will richen the transition circuit and also speed up when the main circuit (primary jets) come in to play.

This O2 kit is a great learning tool. I am finding out why I have made changes to my jetting in the past and not received the change that I was looking for. I thought in the past that the main circuit started around 2200rpms or so (per QF. But this is with the stock setup which mine is no longer.) and now once I chart out the rpms and the O2 readings, I can see the jump in the mixture reading that shows me where the main circuit now starts. I am curious how much lower the main circuit will now start when I change from the .075 IAB's to the .071's. I will show my changes once they are known later in the week.

Thanks for the reply.

Y07_A66,

First, I didn't mean to cause a distraction on your thread. I have read all of these sort of threads in the archives, and was thrilled to see yours. Thanks for sharing the details of what you are doing. It really helps others such as myself to learn.

I do have a wideband and I think a reasonable understanding of carb theory. What I was trying to say was from what I understand the MAB will affect the main circuit activation point. I believe the IAB bleeds air into the idle well, with the idle well getting fuel from the main well via the IFR. As such I don't understand how the IAB could affect the main circuit activation. However, I'm still learning, and appreciate other thoughts on this. There are a bunch of knowledgable folks here, and I continue to gain the benefit of others posts.

In many ways, your are in a much better position than I since you need to add fuel, not take it away. I look forward to your results!


JimG,

I have read many of your posts on carb tuning in the archives, and appreciate your comments. Can you explain some more how the IAB change can affect the main circuit activation? I do have about 20" Hg during highway cruising, and believe even at 3K most of the fuel is still coming from the transition circuit. I didn't mean to get into the details of my setup on this thread (that seems like poor form!), but I would like to better understand your comments better.

How do you limit the transition circuit flow? As you noted, the main circuit is driven by flow. At 65MPH, you may need around 35HP?, which is not a lot of flow. Under these conditions, how can the transition circuit flow be reduced?

The only way I see is to have a greater throttle opening (lowering vacuum), but this will increase HP which will increase vehicle speed.

It seems like the transition is going to be providing most flow under light throttle, and the mains may be delivering just a little. Of course I can lean the transition via the IFR, which will get the cruise AFR into better shape but then the low end of the transition will be lean. I understand I need to get the best balance and shape of the AF flow on each circuit, it is just taking a lot of trial and error!

Thanks for your comments!

Dave

Dave,
""First, I didn't mean to cause a distraction on your thread. I have read all of these sort of threads in the archives, and was thrilled to see yours. Thanks for sharing the details of what you are doing. It really helps others such as myself to learn.""

No distraction at all!! I too am learning about this process and I hope that Jim will use this thread to answer your questions.
I will continue updating this post with my results but Mother Nature is keeping me in the garage this week.

Hi Y07_A66:

DaveNC beat me to it. If you want us to start another thread for this discussion, we'll move on. I don't want to hijack your thread either. At this point, we're just discussing theory that sort of pertains to some of what you're doing. Just say the word and we'll start a new thread.

Hi Dave:

Where are you in NC? We might be next door neighbors. I'm on the southern border of NC in central VA.

Here's the essesnce of the battle being fought by many of us who are trying to tame high HP engines to drive them on the street with good manners at 1 - 2% throttle opening.

You've most likely got a very responsive engine that doesn't need much throttle opening to make the few horsepower it takes to move it down the street (or highway). You've probably got a carb with large throttle plates. Larger throttle plates = smaller opening angle to achieve a given airflow. Many times, this small throttle opening puts your throttle plates smack-dab in the transition slots. Note also that your plates most likely aren't open enough to get the main circuit flowing much, if any at all. So you're cruising down the road operating almost solely on a circuit that is dependent of manifold vacuum, not airflow.

Compounding this problem is the fact that you perhaps have a cam that produces low idle vacuum. So, you get out your box of drill bits and brass set screws and get the Idle Feed Restrictions (IFRs) and Idle Air Bleeds (IABs) sized just right. You've got that sucker idling great with 1 1/2 turns out on the mixture screws; the holy grail according to everybody. Life is good.

Problem is, out on the highway, you're tooling along running pig rich at 20 or 22" hg. Open up the throttle and the carb seems to behave just fine, but you want to drive this car on trips, and this simply won't do.

The problem here is twofold. One, you're driving on the t-slots and not the main circuit. Two, the transition circuit is rich because orfices that are propely sized for 8" hg are too large at 20" hg.

This is why the factory embraced spreadbore carbs when emissions laws tightened up. The smaller primaries allowed greater throttle openings, which ensured that the main circuit was working.

OK, on to your question. Fuel and air for the transition and idle circuits comes from the IAB and the IFR, as you know. After the fuel and air are mixed, there is a "fork in the road". The mixture flows unrestricted to the t-slots, while it flows to the curb idle discharge ports after being restricted by the idle mixture screws.

To limit the transition flow, use the smallest IFR/largest IAB possible and still get a decent idle. Your idle screws will most likely be more than 1 - 1/2 turns out, but as long as you still have adjustability and the idle doesn't suffer compared with what you have today, it's a compromise that we must endure. Once you reach the point where the idle screws have no response, you've gone as far down that path as you can.

Further, any effort that achieves greater idle vacuum allows you to further restrict the idle circuit (IFRs and IABs), thereby restricting the transition circuit. Note that you are still cruising on the transition circuit, but at least you've found a way to lean it out somewhat.

In a perfect world, we'd be able to adjust the length and width of our t-slots. Alas, our world is not perfect.

You can also make and install t-slot restrictors is the main body. This sometimes helps when engines have low idle vacuum/high cruise vacuum, but there are problems inherent with them that usually rear their heads before the problem is totally fixed. I'll elaborate later, but this post is long already.

Back to Y07_A66's results: he's got a very good starting point. He has a carb with annular boosters which is helping the mains get started at lower airflow. Y07_A66, when you get a chance, post your idle vacuum, cruise vacuum, and number, position, and size (if known) of your emulsion bleeds. Based on your data, you don't have any rich spots at light throttle/high vacuum. I'm curious how high your "high vacuum" gets.

Thanks,

J

Jim,
Please keep posting Daves questions on this thread!! Your information helps us both.

Idle hg: 14.5" Neutral @ 1000rpms
11.5" in D @ 800rpms
30mph: 16"hg @ 1700rpms (.075" IAB's = 17.0)
40mph: 16"hg @ 2200rpms (15.8-16.0)
45mph: 17"hg @ 2400rpms (15.6)
50mph: 18"hg @ 2500rpms (15.6)
55mph: 20"hg @ 2700rpms (15.4-16.0)
Main Circuit starting:
60mph 3000rpms (14.8-15.0)
65mph: 17"hg @ 3300rpms (not measured)
70mph: 3500rpms (13.6-13.8)

Sorry, I do not have the emulsion bleed info.
I will update the info above once I test the .071 IAB's so that we have a comparison.

Please carry on.

Jim,

I live in the Raleigh area, a bit over an hour from the VA border but east of you.

I'm certainly in the battle for taming a high HP engine for regular street use. Your description is perfect. My situation is even further complicated by use of a stock 440 torque converter for the time being (started with a MP 175K converter behind my engine, but it was too loose for my taste during normal street driving), and use of a single plane intake manifold (M1) which really complicates the idle situation (more on that in a moment).

I found your comment on spreadbore carbs interesting. Obviously some stock racers can get great performance with ones like the Thermoquad but from a high performance standpoint, I wonder about potential fuel distribution issues with the biased flow in such a carb (ie cylinders 1&2 running leaner). The smaller primaries would be a great solution for my challenges though. Clearly a fuel injection setup would be easier to tune for the various conditions for a street/strip vehicle, but that is not in my budget at the moment! Besides I still feel my carb should be able to work better than it currently is, and I don't overly mind the challenge.

So, back to the theory. Your recommendation to use the smallest IFR and largest IAB possible makes a lot of sense, along with starting the mains early to overlap the transition circuit to cover the lean area. I have already explored that path a bit, and got to a state where I could not cure an off-idle (1200-1300) lean miss and had to come back up on the IFR some. I should probably invest more effort in this direction though but the cam timing certainly presents low RPM mixture challenges. My thinking is that the large IAB was delaying the start of the transition circuit.

I had heard mention of transition slot restrictors before, but don't have a good appreciation of the way to implement this. Also, I can see that it would be fraught with potential problems. Can you explain some more about this?

Currently I have been considering two different directions. One, as I mentioned, was to try to delay the main activation some. However, your comments are reinforcing my thoughts that the majority of the fuel is coming from the transition circuit, and as such delaying the mains would have a muted impact.

The other direction I am considering is switching to a dual plane intake (EBrock RPM). Many seem to be making good power with this. I believe this may help in two ways. First, it should improve the idle vacuum, which hopefully will allow a slightly leaner mixture while still maintaining a stable idle in drive. Second, because each cylinder would only see 1 barrel of primary, it seems to me that the throttle opening position would be increased relative to my current setup, with the benefit of lowering the highway cruise vacuum level, which should reduce the draw on the transition circuit. Would you agree that I could expect both of these benefits from a DP?

Thanks for the thoughts!

Dave

Hi Dave:

You're not far from me at all. Let me know when there's a good cruise-in down your way and I'll come by and we'll hoist our favorite beverages and talk cars.

Regarding t-slot restrictors: look here

http://www.carcraft.com/techarticles/ccrp_0807_holley_carburetors_basics_guide/carb_main_body.html

drill and tap passage "H" to 8-32. Buy some (preferably) brass set screws (McMaster-Carr) and some numbered drill bits, if you don't already have them. Drill an orfice in the set screw as required.

That's the how. Here's the "when should I".

As a last resort!

As I previously stated, this is a trick that's sometimes handy on cars with low idle vacuum and high cruise vacuum after you've reliably determined that the transition circuit is sucking too much fuel at cruise. If you've reached the end of the road with smaller IFRs, and you're satisfied that you've done everything possible to increase idle vacuum (which will allow you to further decrease IFR size) this might help.

I use an Excel spreadsheet to keep track of changes, and I let Excel do the following math. Measure the length and width of the primary t-slots, and get the area. Next, using pi r^2, figure out a hole size that has roughly 75% of that area. That's a good starting point. Do this math every time as a sanity check.

This is a far less than perfect solution, but for those of us working with basic tools in our basement, it's all we've got.

My personal experience says that t-slot restrictors will help in situations of low idle/high cruise vacuum, but not totally fix it. What happens is, the restrictors get small enough that you eventually create an off idle bog at the same 2 - 3% throttle position that's screwing you at cruise, and this bog shows up before the rich cruise problem is totally fixed. Remember my previous comment that orfices sized for 8" hg are too large at 20" hg.

I'll pass on your intake manifold question. Modern single plane designs have surprised me. Maybe you can find someone who has experience with both of the manifolds you're considering on the same engine.

And the big problem with spreadbore carbs (as I see it) is lack of manifold choice. Not much out there, comparatively speaking. But if your car is truly a street car, they deserve some consideration.

Take care, and post your progress.

Hey Y06, how are you coming along?

J

""Hey Y06, how are you coming along?""

I am waiting on Mother Nature to allow me my next test drive. I have swapped out the .075" IAB's for .071" IAB's. I also have .067" IAB's if needed.

Thanks

I got some time tonight to test the .071" and the .067" IAB's. The following show the results:

-.075" -.071" - .067"
30mph 1700rpms 17.0- 16.6 - 16.0/16.2
40mph 2200rpms 16.0- 16.0 - 16.0
50mph 2500rpms 15.6- 15.6 - 15.6/15.4
55mph 2700rpms 16.0- 15.6 - 15.0/15.2
60mph 3100rpms *13.8- 14.0 - 14.0/14.2
65mph 3300rpms N/A- 13.8/14.0 - 13.8

* This could have been a wrong note taken. All of the above tests were done during a windy day. I did my best to keep the throttle even.

.071/.067 notes: 67 deg.
These numbers are after I retuned the carb for best in D idle vacuum.

My light accel at all speeds was 13.8/14.0 with red cam and .037 squirter. This setup should change after I richen up the transition circuit.

My N and in D A/F numbers richened up to 13.6 (.071) and 13.4 (.067), but it seems to like those readings. I had smooth light accel from a stop on each IAB size.

I am still lean on the transition below 2700rpms but above 2700rpms, the 66 primary jetting is looking good.
Is it time to swap IFR's from the current .028" to .031's, or go up to .033's?
I only found three of my .031's, I need to dig some more. Would it do any good to stick the .031 IFR's in the primary side only for a test since I only have three?

Thanks

Jim,

I'm very impressed that you are happy to drive your ride significant distance just for a car gathering....I hope I get my car to that point some day!

Really appreciate the information on the T-slot restrictors. I guess there could be some value to it, but as you highlight it is likely to create a new problem at the low end of the transition. I don't think I'm going to head down that path.

The M1 can accommodate a TQ, but all I have is an old 800CFM one which is too small. I vaguely remember that there was a 1000CFM race unit, but then I have to worry about all of the tuning parts, and as I mentioned, my perhaps not well founded concerns on fuel distribution. So for now, that is also not a path for me.

My intake manifold question is likely one that no one else can answer from a empirical standpoint, since there are so many variables involved. At this point, I am committed to trying the experiment, and will need to gather enough data under various conditions to hopefully be able to come to a definitive conclusion. If I come up with anything interesting in the next month or so, I'll post again.

Thanks again for the food for thought.

Dave