ok... someone explain this to me becuase i feel retarded... from what i've read the air bleeds actualy determine the a/f ratio that is coming out the bottom at idle/cruise...
now, also the idle mixture screws control the amount of this fuel that enters the carb.

so if i needed to richen up the idle i could do it 2 ways, 1 the mixture screws to let more "mixture" in at idle and cruise or 2 change the size of the bleed to richen the "mixture".

so what if i need to keep my idle the same, BUT richen up the off-idle cruise how could i ?

Reason i ask is i have it set perfectly at idle, BUT when cruising on the highway at 1800 or so (basicly the front blades are just cracked open) it goes totaly lean and stays there if i hold the throttle in the same place that's just above idle (it's 1 spot only and with just a tad bit of load like on the highway or a slight incline). If i richen up the idle mixture screws at idle, this problem is gone, but it's pig rich at idle and tends to foul plugs...

so, would using the air bleeds to lean out the idle help or change it thru out the idle/cruise circuit?

i'll be getting a wideband soon but just wanna get this clear before i go to tuning drasticly.....

ANY info on just tuing the idle/cruise circuits would be great! not the pv or main's as i can deal with that....

im using a narrow band meter to see this happening FWIW

What carb are you running?

LOL, guess that would help 830hp mech. sec. with anular boosters

Ok...

You have an idle jet that controls how much fuel gets into the idle circuit. And one or two (depending on carb) idle air bleed jets for the idle circuit. These both add air to emulsify the fuel mixture and bleed off some vacuum signal. It is this bleed off affect that changes the timing and mixture with bigger air bleeds leaning and smaller air bleeds richening the mixture.

Now the confusion you have regards the fact that the idle circuit has two different outputs: the curb idle port and the transfer slot. At the lowest idle position the transfer slot is basically open to the high pressure side and not to the manifold vacuum. Thus it works as yet another air bleed and all fuel mixture must pass through the curb idle port. As the throttle blades are opened the transfer slots progressively move from the high pressure side to the manifold vacuum side. As they do this they change from air bleeds to fuel mixture draws providing more fuel to match the increasing air flowing around/past the throttle blades.

The idle mixture screws only control how much mixture can pass through the curb idle port. Note the use of the term "mixture" and not fuel as this is an important distinction.

The other affect is that the more vacuum signal is bled off by the idle air bleed the more vacuum is required to pull fuel through the circuit. This has two important affects both based on manifold vacuum dropping below this threshold. The first is on the low end with an off idle stumble and the second is on the high end when the main circuit takes over. So this means that if you lower the vacuum threshold with smaller idle air bleeds, you also prolong the upper end of idle circuit operation too.

Now let's apply some of this theory... say your off idle, transition, and cruise are perfect but idle is off. You would adjust the idle using the idle mixture screws. But should your idle be perfect and your transition and cruise be off then you would have to use a combination of the idle fuel jet and the idle air bleed to adjust the mixture.

So in your case I might first open up the idle jet a bit to richen cruise since you appear to have no other adverse affects working. You will have to also close the idle mixture screws a bit after this.

Steve

what Steve said....

Wow... this thread died out quickly. Was it something I said? ;-)

chev-hell, I hope you update this thread with your results. I would be very interested in seeing how changes compare to your starting point.

Steve

well, I wrote up a nice long reply to this, but fell into the great bit bucket in the sky somewhere. I'll re-post later after I'm not so disgusted.

I have had that happen once or twice lately too... major bummer.

very good writeup vrooom. thanks for taking the time to do it. excelent breakdown of the relationship between the curb idle discharge and the transfer slot. if the fuel available to the entire idle circuit is increased it will fatten the transfer, but the idle mixture screws still work to lean out the curb idle. a couple points: the idle jet is known as the idle feed restriction, it's a drilled orifice in the metering block, not something you can change with a screwdriver. some blocks have this right out in the open, on some it's internal and inaccessible to the regular guy working at home. the trick is, the hole is very small and increasing it just a few thousandths changes the metering area a lot. it's real easy to drill yourself right out of the ballpark. metering flow depends on the area of the hole, which increases with the square of the radius. you don't want to drill it more than about .002 without testing. of course, you've got to be able to accurately determine the existing size before drilling anything. this is a place where a wise-band O2 sensor is almost required.

There is a relationship between the size of the feed restriction and the idle air bleed. in order to avoid unexpected results it would be wise to measure the air bleeds, calculate the area because if the feed restriction changes very much the air bleed will need to be adjusted too. Don't ask how I know this, but having 25 years background in this stuff means a guy has made alll the popular stupid mistakes at least a couple times. :)

Thanks again

Tom, this sounds like something that the home mechanic shouldn't be tinkering with. Do carbs shops do this sort of thing...like lean out the idle/cruise mixture for a 14.7 afr? That would probably save us all a lot of gas (since performance carbs seem to be in the 12.5 cruise range).

Vince,

In order to get picky about the A/F ratio you just about have to have the car on a chassis dyno and use a wide-band O2.

The 14.3 deal, that's for smog engines engines mostly. Most old-fashioned performance engines won't run well that lean. Some won't run at all that way. In order to make that work well you'll need computer controlled fuel injection and ignition.

the idle circuit can be fattened up a little if the feed restriction is accessible, but you just have to be careful and go a little at a time. in this case, an 830 DP Holley is a race carb, pig-rich everywhere. I'd guess that it's too big for the engine it's on or some tuning issue exists that hasn't been sorted out yet.

Tom, I do have a LM-1 setup, and I can get to about 14.7 before I get a lean-miss. I guess this is b/c I have a very mild (almost stock) engine. I think I'll be headed to a chassis dyno at some point just to compare notes and find some power.

sorry guys, i've been extremely buzy... 3 birthdays, anniversary,thanksgiving all in a 2 week spand... Im also installing the March serpentine set-up on the car so, i'll have to wait a couple weeks till i can get the wideband to tune it, but that's the greatest write-up so far, not even the books spell it out that clear.

...The 14.3 deal, that's for smog engines engines mostly. Most old-fashioned performance engines won't run well that lean. Some won't run at all that way. In order to make that work well you'll need computer controlled fuel injection and ignition.
I am not convinced that EFI is a requirement. Apart from the closed loop feedback and adjustment, part of what distinguishes EFI systems is the even distribution of fuel into each cylinder. If this even distribution can be duplicated with a carb you should be able to run it just as lean as EFI.

With older manifold and carb designs you wind up with more fuel in some cylinders and less in others. You have to run richer so that the leanest cylinder does not go into a lean misfire.

There are two design elements that will help a carb even out mixture distribution: high venturi air speeds (think spreadbore) and annular boosters. Both work to more completely atomize the fuel up front. By doing this they reduce the size of fuel droplets and the drops then tend to follow the air flow more closely since they have less inertia.

As to ignition, a longer spark and/or multi-spark will assist in making sure the leaner mixture gets fired each time.

Glad the write up helped. I have been doing a fair amount of reading and analyzing myself recently to understand the mismatches between my Edelbrock Performer 750, Air Gap manifold, HR296 cam, and 402 BBC. That is part of why I used more generic "idle jet" terminology instead of the Holley "restrictor" term. Although they use different terms most of these carbs all work the same way.

Fascinating devices these carbs and engines. I plan to post more about my experiences and results when I have some quantitative data.

Steve

It's possible to put some small wires into the idle air bleeds to richen up the transfer. You just have to be sure you do it in some way that they can't fall out.

Here's a good article from CHP, might be helpful
http://chevyhiperformance.com/tech/engines_drivetrain/induction_poweradders/0510ch_tune/

Turbodave's link should be read by everyone running a carb. Only after becoming aware of the transition circuit can you begin to adjust it. Having the right transition mixture is the reason that some people think that a Holley is better than an Eddy, is better than a Q-jet. All carbs are good, and all are tuneable. Knowing how to adjust them is more important than the name stamped on the side of the carb. ( IMO the Carter/Edelbrock has the easiest transition circuit to work on. Nothing is hidden inside of anywhere. All holes are easy to enlarge or restrict and there is no fuel spillage when adjusting. Idle restrictors are easy get to. And don't tell me it's not as tunable overall, as some Holleys. Holley has more options available at WOT.)

Another thing that people that are new to racing don't understand is that nobody races dynos. Dynos stand still, cars don't. Acceleration has a great affect on mixture strength from front to rear on a drag car. When you come off the line hard, the gas in the manifold is trying to push to the rear of the manifold the same as your butt is trying to go into the back seat. That is the reason computers are so good when it comes to track tuning. Thermocouples on each cylinder read the exhaust temp and tell where you need to richen or lean. Don Garlett's didn't break into the 200's until he pointed all the injector nozzles to the front of the blower, to get the front cylinders running richer. That is what led to putting most of the fuel into the cylinder runners and less into the top of the blower. The most important thing in getting any combination of parts to work is the ability to understand how to read plugs and give the individual cylinders what they are asking for. Once you know what to do and how to do it, running good gets easier. Any dyno testing is a good start, but you may still need to vary track jetting depending on YOUR setup. Take notes and don't be afraid to change stuff.

The reason I stress plug reading is that exhaust gas analyzers only test the bulk amount of gas, not the individual cylinders. You must treat an eight cylinder engine as eight one cylinder engines. That's why modern builders stagger jetting, rocker ratios, and even cam lobe placement, try to heat some cylinders and cool others. All because of trying to get all cylinders to run the same. And it works.