As a general rule, I have read for years that both the Primary side and the Secondary side floats should be set at the same level.

But, would adjusting one float higher than the other make certain parts of the fuel curve richer while leaving other certain parts of the fuel curve alone? In other words, could adjusting the Primary side only affect only the transition and main jets while adjusting the Secondary side only affects WOT A/F?

Can you use the Primary and the Secondary fuel levels to fine tune only one area of the curve?

I assume that one is going to affect the other, but I am curious.

I have seen the affects on my O2 gauge when adjusting both bowls higher or lower, but I have not messed with only adjusting one float at a time to see what changes occur.



Thanks

i'm going to go a little bit out on a limb here. i'd use the air bleeds to effect the fuel curve not the float level. too high of a level will richen things up. too high of a level in the secondary bowl can cause fuel spillover during hard braking with a holler type carb.

""I'd use the air bleeds to effect the fuel curve not the float level""

I am about as close as I can get with my bleeds and now I am looking for an external adjustment that can be tweaked before opening up the carb. If I could tweak .2/.4 on the A/F gauge by turning the Primary or the Secondary float by 1/8 or 1/4 of a turn, then I might try testing that and see what happens. But if adjusting one float only screws up too many other things, then that would not be the way to go.

Thanks for the response.

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Does the Primary bowl fuel level affect only the Primary circuit?
Does the Secondary bowl fuel level affect only the Secondary circuit?

If so, then I should be able to tweak the P or the S circuit slightly by only adjusting the corresponding float. Is this correct?

I don't like to set my floats any lower than the bottom of the glass and I don't like to set them any higher than the middle line. If this is correct, then once the carb is setup fairly close inside (jets, pv, internal bleeds, etc.), then I should be able to use my float adjustments to fine tune the A/F of the P circuit and the S circuit "separately" as long as my float levels are in between the bottom of the sight glass and the middle line in the sight glass.

I would have to reset my idle A/F mixture after each float adjustment, which is no biggy.

What else am I missing?

Thanks

Oh man

I've read and contributed to some of your tuning threads before and I have learned from experience here to watch out asking detailed holley questions-

for example:

Before I knew how to tune using idle feed restrictions nobody had any suggestions about how to lean out the off idle cruise performance. After I had done some research and tuning with wires etc (coulda bought HP blocks, but Im cheap) I saw major improvement and THEN started hearing about "well the first thing to do for a street driven double pumper is limit the IFRs until the idle mix screw (a misnomer I now know) is correct around 1.5 turns"

It's like when you learn a new word and start hearing and seeing it everywhere right after, except around here if it's a hard word no one wants to teach it to you- they only wanna debate pronunciation after you already know the spelling and definition!

Sorry for complaining- but I kinda doubt you'll get the answer you're looking for. Probably lots more 'try the air bleeds first'. I'm not trying to flame anybody's answers above, they're just trying to help and most Qs around here aren't really looking for an in depth answer.

Having said all that, I know you are chasing a perfect tune with the least amount of compromise possible. I've read some of your posts and watched you learn and experiment. I am no guru or 20 yr veteran tuner (you're way ahead of my efforts already) but I do know that once you know all the user tunable stuff (jets, power valves, acc. pump stuff), and the 'expert tuner' stuff (air bleeds, idle feed restrictions) you might still have to hack at it some (wires or custom jets to tune power valve channel restrictions, drilling butterflies) and sometimes that stuff still won't get you the efi-like drivability you're after. At that point you should be chasing perfection because if you still don't have the basics down and you're hacking up a good carb you're just going deeper into the woods.

I think you are at that point where the magical and mysterious science of fluid dynamics goes way way past my education and gets into the realm of tinkering and datalogging with a-b-a testing. I think you should try different float settings out- what could it hurt? The level is definitely related to the drilled emulsion holes and all that fancy stuff on the metering block. You probably know that some 'trick' blocks and some stock blocks have different patterns of holes. I don't pretend to understand it well enough to explain it, but on the most basic level I'm sure that a higher float will make a richer mix. Once you get away from the stock level you run the risk of getting into other problems that can't be tuned out, but that's all part of the fun right? Then you drive up into another elevation, or weather/temp change and everything is all wrong all over!

I said my 750 DP was good enough when I got good WOT/launch, decent manners and mileage (sorta), and don't foul plugs in traffic, but I have MUCH respect for your efforts. Sorry for writing a novel here- Good luck with your tuning I'd be interested to hear how you make out.

rdr

I have messed around with adjusting float level to enrich or lean out the fuel curve. Basically what you are doing is covering up or uncovering the emulsion passages on the metering block, which in theory will change the "tilt" of the curve. This is a good explanation on the subject:

http://www.quickfueltechnology.com/technical-info/emulsion

My experience has been that if you drop the float level down to the point to leaning the transition it will be too low and potentially cause fuel boiling issues, high speed starvation, flat spots while cornering.

On the contrary if you have the float level high enough to enrich the circuit you can experience spill-over, fuel siphoning, or dribbling booster syndrome.

This is the reason they sell emulsion bleeds. If you have the air bleeds as close as you can get them you could make very small changes in float level and see if you can sneak up on it. Most of my testing has been at 4200' altitude so your mileage may vary.

Let us know what you find.

Set them as high as possible and index the needles and seats to get the best fuel inlet into the float chambers and leave that alone. Use slosh tubes on both the front and rear vent tubes to prevent any sloshed fuel from going into the main body Tune from there,one circuit at a time, keep the varaibles to a minimum Remember, a Carb. is a simple device that is sensitive to weather changes, it is not computer operated or EFI

I have allot of fun tinkering with my carbs. All I know is that the more I tinker, the better the car runs and the more I learn. The O2 sensors are very addictive and a great tuning tool.

I am still looking to see if anyone has messed with tuning each bowl level seperately instead of keeping them both set at the same level.

I will start messing with it after my next test drive to see if small increments (1/12, or 1/8 of a turn or so) will tweak the main or tweak seconds in the direction that I need.

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Does the Primary bowl fuel level affect only the Primary circuit?
Does the Secondary bowl fuel level affect only the Secondary circuit?


Thanks for the responses.

i'm not trying to be a smart a$$ but many people don't understand the basic mechanics of fluids that a carburetor works on. technically there's no such thing as a vacuum only an absence of pressure just as there's no such thing as cold only an absence of heat. carburetors operate on pressure differential. yes adjusting the float level can/will influence the mixture but imho that is not something i would use as a tuning tool. without taking a quick look at a metering block i don't recall what you may or may not uncover but changing the level will vary the static head of the liquid so it will take greater pressure differential to draw it into the carb. i would expect this effect to be a very slight change. fuel is not "sucked" into the engine it's pushed in by the atmospheric pressure present in the fuel bowls because the venturi and engine create a lower pressure area. personally i think carburetors do a pretty respectable job for what they are. yes the bowl levels only affect the circuits on the side of the carburetor they're on.

My two cents....

The primary and secondary circuits in the carb are independent but may both affect fuel distribution in the manifold. Leaning/richening one side of the carb to get to a specific AFR on a gauge might lead to some cylinders being out of balance. Determining cylinder to cylinder AFR with a wideband is not really possible from what I understand. You may see some fluctuation but the sensor is always doing some measure of averaging.

I don't see anything bad about experimenting in itself. Persistence and willingness to explore can lead to new insights. However, it is important to arm yourself with as much understanding as possible in order to look out for and understand the potential consequences with any change. There are many good points in the other posts here.

Small changes in float level can have an effect on the AFR over the entire operating range of the carb, although perhaps not to an equal degree (ie idle, transition, main). Because of this common wisdom is to set float level first, then tune the different circuits. If float level is later changed, then some of the other tuning items may need to be tweaked again.

Tweaking the idle is easy but not so on the other circuits. If you are looking to adjust just a portion of the fuel curve, float adjustments may produce nothing but frustration.

That said, I have never tired to set the P and S floats differently.

""yes the bowl levels only affect the circuits on the side of the carburetor they're on.""

""The primary and secondary circuits in the carb are independent but may both affect fuel distribution in the manifold. Leaning/richening one side of the carb to get to a specific AFR on a gauge might lead to some cylinders being out of balance.""
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Thank you both for these responses.

I always try to get both bowls as close I can to each other but they have never been exact. If they are both close to each other, than I call it good.
The adjustments that I am considering would only be within a half of a turn or so from each other since I am so close to the numbers that I want.
This is a consideration due to that fact that the floats can be adjusted in minute distances due to the threaded adjusters on the needle/seat assemblies.
I might give it a try just to see if I can move the A/F gauge by .2-.4 or so just on the primary side or just on the secondary side and note what happens. I also understand the changes that the weather creates on the A/F numbers and this is another reason why I am considering using the external adjustment of the floats.

Thanks allot for all of the replies.

Yes people have.
If you dig around that Innovate forum you'll turn up discussions of it. You're a member there IIRC so you can use that board's search function. Try searching for sight glass or variants of that as well as float level. I recall Tuner in particular writing about how Holley had different hight settings and matching bowls for production carbs. IIRC there is a sentance or two in the Urich & Fisher book as well.

Agreed that fluid dymamics get really tough when the fluid is changing from single phase to two phase. That's been the hard part for me to get my head wrapped around.

Radar, I basicly agree with you, but I figure that's a part of the new information age. It's up to the end user to filter the information, and its somewhat random whether the right person will see, understand and respond to a query (or even if a search will turn up useful information). There are things you can do to increase the odds, but that seems to be as good as it gets. You know, there are places where signal to noise ratio is better on certain topics, and places where its worse.

Yeah, I've set the secondary float a bunch lower to lean out the secondaries on a road trip and it seemed to help.
The carb was an 1850 with a metering plate in the secondaries and at 6000 feet it was fouling plugs. Lowering the float seemed to help.
As for fluid dynamics, best power seems to be when the fuel is not vaporized but instead is in very small droplets as the mixture enters the cylinder. Vaporized fuel takes up space that could be more air.
And, lowering the fuel level reduces the head on the jet so less fuel flows.

R.

The three most dangerous words,"I have heard."

I have been playing around with the float levels again and I have found that I can "tweak" the cruise A/F with the front float level and I can "tweak" the WOT A/F with the secondary float level. I do have to reset the metering screws after the float setting changes to keep the in-D A/F the same.
I started both float settings in between the bottom of the sight glass and the halfway mark (apx 1/3 up the glass) while getting my cruise A/F and my WOT A/F tuned. Now what I have found out is that as the weather changes, I can adjust the A/F numbers by just turning one float or the other by as little as about 1/12 of a turn. Our weather changed 30 degrees in one week, and all I did was about 1/10-1/12 of a turn to each float and my A/F readings fell back into place. Keep in mind that I am cruising on the edge of lean with my combo. Once my cruise numbers rise past 15.0, then I readjust the front float up and my numbers fall back into the 14's where I know that I am safe. If I went into the low 14's at cruise, then I might not have to worry about the weather change so much, but since I now know what it takes to correct it, I just make a quick change before I start up the car when the outside temperature shifts in large amounts, then I am ready to go.

Note: I can do this now that I have my cruising and WOT A/F numbers where I want them.

Note: I currently have both floats about the same height. They are both about 1/16" above the bottom of the sight glass's and my metering screws are all still equal. Even if they were a "little" different from front to back, I can't see why this would hurt anything as long as the A/F numbers were coming out where they need to be.

Changing the fuel level is a quick and dirty way to make a change to the fuel curve. There is no rule that the primary and secondary fuel levels need to be the same. You just need to be aware that when you change the fuel level you're making a change to multiple circuits at the same time. When you change everything at the same time it can confuse the results so you have to keep a clear head and pay attention to the data.

You're trying to find a .2 adjustment on the A/F curve with a carburetor?

The easiest way to do it is to wait for the weather to change.

Once you get it perfect, the temps will increase, humidity will change, and the barometric pressure will increase.
When that happens you get to do it all over again.

If you must have perfection then go EFI. Should that not be an option be prepared to retune the carb after every 8 degree change in temperature or 10% change in humidity.

.2 is not enough for me to worry about but adjusting the floats by 1/10 or 1/12 of a turn is a quick way to gain .4 or so if needed as the weather changes drastically. To those of us who want to run carbs & O2 sensors, this is an easy way to make the adjustments per the weather. I am not expecting the perfect universal tune. I am just showing that small adjustments can be easily made without tearing into the carb every time the weather changes.

wouldn't the effects of fuel level only be accurately monitored in a perfect world (On a dyno)where the fuel in the bowls isn't being sloshed around from braking ,accelerating or cornering?

the only way to get "the perfect mixture" would be on a dyno with o2 monitor/sensor