Carb Float Height, for Beginners

[opus10583]

Hi,

Can someone please give me the Carbs 101 explanation of how carb float height affects carb operations?

Thanks,
Mark

[BEEK]

The float level setting is absolutely critical because it effects the fuel mixture throughout the entire operating range of the carburetor; too high and the mixture is rich, too low and the mixture is lean. Regardless of engine speed or load, the float lets fuel into the float bowl at the same rate it is being used, so the fuel level within the carburetor float bowl remains at a constant level.

[majicwrench]

From a technical point of view, tis nice to have float set to factory spec, as it can make engine leaner and/or richer. In theory, having a deeper pool of fuel increases the pressure at the bottom of bowl, where jets are, thus more fuel. ANd if float too low, less fuel= less pressure at jets. Also, if too much fuel in bowl, increases issues with hot-soak starting issues.

BUT really, having the float a bit off spec is NOT going to make a noticable difference. Try to imagine the pressure differece .050" +/- is gonna make in float bowl. Way too high and is gonna flood, way too low and is gonna starve. Close to spec is gonna work fine.


Again, nice to have it set to factory specs, that is why we get factory specs.

[racydave]

If the float is to high, fuel will drip over in to the carb at idle and is visible, especially while looking with a timing light. Too little in the bowl, and it may occasionally starve. Rule of thumb, the float may be set just a little less than level. Richer or leaner? No. The volume of air controls this. Along with the jet size.

[majicwrench]

So you don't feel fuel dripping into carb is going to make it richer?? And starving is not going to make it leaner??

Like I said, from a truly tech point of view, it will affect the mixture. But slight changes in float level are gonna result in insignificant changes in fuel mixture.

"Rule of Thumb", I avoid those , why guess?? Why not find the correct spec, tis easy to find.
Keith

[BEEK]

dave, i have to disagree with you on that one. it defiantly effects richer and leaner, it goes into the area "is close enough good for you" if kinda works, might be rich, might starve, kinda runs funny. all of these are in the ok scenario. but you might be trying to find a running problem and if things are not to spec, then how do you know what is correct and what is not. specs are published for a reason, not because the manufacturer wants to be in the printing business

[DanD]

dave, i have to disagree with you on that one. it defiantly effects richer and leaner, it goes into the area "is close enough good for you" if kinda works, might be rich, might starve, kinda runs funny. all of these are in the ok scenario. but you might be trying to find a running problem and if things are not to spec, then how do you know what is correct and what is not. specs are published for a reason, not because the manufacturer wants to be in the printing business

Yes, but in general you set it and forget it. It is important that the float pivots correctly and the needle valve can control the flow of fuel. Unless you have a window in your float bowl, you are guessing at the final fuel level in there, so yes, we strive to be accurate, and make sure the parts are adjusted properly and functioning correctly. After that we tune to conditions.

Since we are on the topic, having the right fuel pump, volume of delivery and fuel pressure in a certain range is also very important. It all works together as a system. If you have fuel spilling over the bowl, some thing, or things would be wrong. If you are winding out in third and you have a sudden loss of power because the carb has been sucked dry, something is wrong. But generally you address the system as a whole, and it will function as designed. But once the car is running, and you are driving the car as it was meant to be driven, the fuel bowl is not a terrifically static place that is 100% level and controllable to the milliliter, as in, Fling It Around Turns. So yes, we strive for perfection, and then we deal with reality.

[racydave]

Obviously it does affect richer or leaner, but if you are within the margin allowed it doesn't. My bad... And the rule of thumb applies, after the float is set to specs, it is a good check to confirm the level. A wrench would fit in my mouth better than words.

[opus10583]

Hi,

Thank you all for your input.

I'm still at a loss to understand something; where's fuel spilling over what if the float's set too high, what's not getting fuel if the float's too low?

Thanks again,
Mark

[majicwrench]

If fuel too high becomes easier for vacuum to pull it from bowl to venturi, thru jets. IF gets way too high, will start spilling everywhere, out of carb and into barrel.

If too low, jets will start to become uncovered on corners etc. and is harder for vacuum to pull (or more accurately, for pressure to push) fuel thru jets and up thru venturis.

When drinking thru a straw, as level of drink drops in cup, the harder you have to suck to get a drink.

[opus10583]

If fuel too high becomes easier for vacuum to pull it from bowl to venturi, thru jets. IF gets way too high, will start spilling everywhere, out of carb and into barrel.

If too low, jets will start to become uncovered on corners etc. and is harder for vacuum to pull (or more accurately, for pressure to push) fuel thru jets and up thru venturis.

When drinking thru a straw, as level of drink drops in cup, the harder you have to suck to get a drink.

Thanks!

One more, please: If the float is set too low, drops too much, doesn't fuel just flow into the bowl unimpeded?

That is; once off the needle valve, what does it matter what the bottom limit of movement is to the level of fuel in the bowl?

Thanks again,
Mark

[majicwrench]

Not sure I understand question

[131]

Not sure I understand question

I think he's referring to the bottom limit of the float, needle valve fully open.

[opus10583]

Not sure I understand question

I think he's referring to the bottom limit of the float, needle valve fully open.

Hi,

Yes, thank you! ...I was trying to find a picture but that's it.

Thanks again,
Mark

[majicwrench]

Well, I still don't understand what you wish to know. Certainly, when the needle is not seated, fuel flows, but methinks you know that.