Carburator floats , what is their Real purpose ?

[Bob Kelly III]

In another Thread a fellow member whom I have allot or respect for maintains that the carb floats are not meant to be a shut-off valve for the gas....
and I totally disagree.... that is what they were meant to accomplish, and if their not accomplishing that task their not set correctly .... to me it's very straight forward....
However I would like to hear More Opinions on "the theory of Operation" of the floats in your carburetors ! .....
if you agree or disagree with My view I would like to hear it....
and a description as why you think that way would of course add to the validity of your idea.
....
I found that setting the floats BY THE BOOK failed to stop the flow of fuel through the carbs....( with a full tank of gas) , I had to re-set them Just a bit, to get them to stop the flow completely.... perhaps this is the reason this theory has started, ... setting them by the book doesn't work perhaps ????
..... in every carburetor I have ever worked on the function of the float and float needle Is to shut off the flow of fuel completely.... not just partially..... Completely it this was not the case then you would have gas escaping the carb in an Over flow condition.... not what any of the manufacturers intend I am sure!
the only thing I can think that would confuse the idea is the fact that they put a petcock before the float bowl so you can turn the fuel off, after all you don't have that in a 1970's era car or truck why do you have one on a motorcycle ?
My answer to that is that the car or truck are fuel pump fed at a fairly constant pressure and the floats can be set to shut them off very well and when the motor isn't running the gas isn't trying to push the floats down.... that is not true on a motorcycle, when a bike is setting with the fuel on the entire weight of the gas is setting on the float needle in the form of pressure all the time 24/7
however that pressure is relitively small, BUT it is still there, so as a safety precaution they put a on off valve to stop that pressure ......Just incase one of the floats stick open which they are prone to do
in any carburetor because of impurities in the gas. and with a motorcycle you can park it in a very un level angle too... which can aggravate the situation...... on the side stand on a steep hill for instance.
....So I maintain that the Reasoning of the Float and float needle is to shut off the fuel flow completely.... not partially , as fuel would still leak out causing a danger.
Let us Increase our Knowledge here shall we ? shed some light on the subject and maybe we will all come out smarter in the end !.... I hope to anyway ! <GRIN>
Please add your comments !
Bob..........

 

[TwoManyXS1Bs]

Here ya go, Bob. Grind thru these first:

http://www.xs650.com/threads/fuel-in-the-oil-xs2.39767/

http://www.xs650.com/threads/carb-question.47807/

 

[weekendrider]

Because of components, you need to access by removing the tank? If you did not have a petcock or a valve of some sort that could get tricky.
The tank valve isolates fuel in the tank.

The carburetor operates best with the fuel level at a certain point. Too low and it is lean, too high it is rich.
The float valve isolates fuel in the carb bowl.

 

[Bob Kelly III]

2M.... the last statement you made on the last thread you posted makes the most sense ! however What I saw was a few screw ups in statements then everyone saying the same thing
but that statement by RG is what I thought WER was saying ! and in that thred he said just the opposite so there is still some ambiguity or uncertainty here I think......or just that I've been reading too much ! LOL and can't keep it straight ! LOL
I don't think anyone misunderstands HOW the system works in the 2 threads you pointed me to... the problem is the wording
and thinking that the float valve can and does act as a regulator for the fuel level but doesn't act as a Shut off valve.... and I have to agree with WER on that.... it most certainly does.... albeit a rather iffy one at times.... it's Supposed to be a good one ! LOL........
......
what I want to know is..... does anyone think that in the carb float bowl is a system that doesn't shut off the fuel ? if so someone must learn some more..... either that person Or me... because I think it most certainly does. it's not a Perfectly flawless shutoff valve Granted !
but a shut off valve nonetheless .
WER.... I totally agree with your statements in the 2nd thread 2M pointed out, so there is no need to go into that here I guess,
but that isn't what you said to me on MaxPete's build thread..... I got the impression You and others were of the persuasion that the float and needle valve in the carb was never designed to shut off the fuel..... and that set off a Red flag warning in my head ! but evidently its the wording... that is the problem , not the understanding of the mechanism !
.....
Bob........

 

[xjwmx]

It's ok if it leaks, as long as it wouldn't leak faster than the engine is burning gas. But when the engine is off, it's burning no gas...

 

[mrtwowheel]

NO, it's not OK if it leaks. Maybe your bike has a "leak meter", my bikes are not so equipped.

Scott

 

[TwoManyXS1Bs]

Yep, the wording can bite ya in the a$$. 'Specially after a big dinner, on a stoopidphone, with its own peculiar spellchecker.

The float and valve's main function is to maintain some sort of fuel level. They will cut off the fuel entirely, but that's consequential, and a handy place to establish the fuel level. While the engine's running, there'll be a flow demand, so they're actually a part of the metering system. The jetting system expects fuel at that level.

At a typical cruise, you'll be burning maybe 1.2cc of fuel per second, that's 0.6cc per carb, which works out to about 12 drops of fuel per second. Take an eyedropper and dribble 12 drops per second. That's your fuel flow rate thru the carb. So, the float valve is actually metering the fuel replenishment to the bowl. From all the jostling and splashing it may cut off flow, but only momentarily.

At engine shutdown, the fuel demand stops, and the float will rise to cut off the fuel. A nice thing to have, otherwise you've got a mess. But, that's within the human perception timeframe. Over the longer period, the fuel boils out and/or evaporates out. All the float valve system knows to do is maintain the fuel level, and it'll let fuel back in to replenish what's lost. That's different from a leaking valve, which will allow overfilling.

So, you need a positive system to cut off fuel. For the old cars and trucks, the fuel pump stops. For a gravity feed system you need an isolation valve.

 

[xjwmx]

NO, it's not OK if it leaks. Maybe your bike has a "leak meter", my bikes are not so equipped.

Scott

It's okay as long as it leaks slower than the engine burns it. To visualize -- say you have a perfect float valve that seals perfectly. Now say there's another way for gas to get into the bowl, say a little pinhole. As long as gas isn't going through the pinhole faster than the engine is burning gas, the float will still keep the level in the bowl where it's set to be.

 

[mrtwowheel]

Yep, the wording can bite ya in the a$$. 'Specially after a big dinner, on a stoopidphone, with its own peculiar spellchecker.

The float and valve's main function is to maintain some sort of fuel level. They will cut off the fuel entirely, but that's consequential, and a handy place to establish the fuel level. While the engine's running, there'll be a flow demand, so they're actually a part of the metering system. The jetting system expects fuel at that level.

At a typical cruise, you'll be burning maybe 1.2cc of fuel per second, that's 0.6cc per carb, which works out to about 12 drops of fuel per second. Take an eyedropper and dribble 12 drops per second. That's your fuel flow rate thru the carb. So, the float valve is actually metering the fuel replenishment to the bowl. From all the jostling and splashing it may cut off flow, but only momentarily.

At engine shutdown, the fuel demand stops, and the float will rise to cut off the fuel. A nice thing to have, otherwise you've got a mess. But, that's within the human perception timeframe. Over the longer period, the fuel boils out and/or evaporates out. All the float valve system knows to do is maintain the fuel level, and it'll let fuel back in to replenish what's lost. That's different from a leaking valve, which will allow overfilling.

So, you need a positive system to cut off fuel. For the old cars and trucks, the fuel pump stops. For a gravity feed system you need an isolation valve.

Seems to me that fuel loss would happen if the petcock was closed or not, it would leave from the tank. With the float bowl always full there would be less buildup of gum. Less chance of valve or float failure.

Yes, I treat my fuel and leave the carbs full and turn off the petcock when storing my bikes. I start them with new fuel and run a lot of treated fuel through them at first startup to clean out any gum that formed in the carbs. If my float valves are going to fail it will be in THIS short period of time, also, if my float is going to stick it will be in THIS short period of time, not when I am actively riding my bike through the season.

 

[mrtwowheel]

It's okay as long as it leaks slower than the engine burns it. To visualize -- say you have a perfect float valve that seals perfectly. Now say there's another way for gas to get into the bowl, say a little pinhole. As long as gas isn't going through the pinhole faster than the engine is burning gas, the float will still keep the level in the bowl where it's set to be.

You should leave your petcock on more often so you can realize that YOUR CARBS ARE LEAKING !!!!!!!!

Scott

 

[Bob Kelly III]

xjwmx: in that last scenario all is fine Until you park the bike and leave the fuel on..... then you have an overflow condition ...Correct ?
.....
Bob......

 

[Bob Kelly III]

So, you need a positive system to cut off fuel. For the old cars and trucks, the fuel pump stops. For a gravity feed system you need an isolation valve.

2M: Agreed, what your calling the Isolation valve on the gravity fed system is the petcock on the bike Correct ?
..... I must admit I didn't know that evaporation, condensation and a long period of time was that serious of a problem.
I remember years ago not starting my Bike for at least a week in the hot summer ( that was the time I got sun blinded.) I went out and tried to start the bike and got a few sputters but that was all.... I thought what the heck , and ground the electric starter till the battery was almost dead.... and THEN.. i discovered that I forgot to turn on the gas ! LOL I turned on the gas ...and waited a minute or so shaking my head at what a dummy I was, hit the starter and it fired right up .... but that was a Hot summer and it was parked in a place where there wasn't much if any shade at all.... that was an instance of All the fuel in the bowl evaporated I am sure ! but that was kind of extenuating circumstances I thought !
I wonder how long it would take to evaporate a float bowl of fuel in the hot summer sun ? I'ed think just a few hours, so why did it take a week before I ever noticed the problem ? recondensation I suppose.... it probably took a week for all the vaporized gasoline to escape through the Idle circuit in the carb. or it may well have been completely dry the 2nd day , who knows
I do know that gasoline, regular Unleaded gas, will All flash to vapor at a mear 110 degrees ambient temperature.
at 115 degrees in the shade you can take a small can of gas and set it in the sun and watch the level go down ! it's slow going to vapor because it is colder than that ! as it evaporates it cools itself...... making running on Vapor ONLY a big problem !
I made a 3.5 B&S engine run on vapor only, fairly simply took a steel container, sealed it up but added a pipe down the center so vacuum from the engine would bubble the gasoline.... added ball valves to it one on the tube and one as the throttle on the intake
it would run fine when you got the mixture of the 2 valves set correctly..... I put the exhaust pipe through the container to heat it up but it was NOT enough the evaporating gas in the container got so cold frost formed on the outside of the container..... but it ran great.
I only got about 4 times the normal fuel milage out of the conversion though and never did find a way to link the valves together !
..... an interesting side note anyway.... lol
......
Bob.......

 

[TwoManyXS1Bs]

Seems to me that fuel loss would happen if the petcock was closed or not, it would leave from the tank..

Yup. Hence the EPA's "Evaporative Control" systems.

...With the float bowl always full there would be less buildup of gum. Less chance of valve or float failure...

With the old daze fuels, if the petcock were left on, and fuel were allowed to replenish the bowl, the gum buildup increases. What evaporates off first is the light volatiles, leaving behind the heavier organics. That stuff slowly oxidizes, becomes rancid. The cycle continues, lights evaporate, heavys remain, the goo zoo. Even with the petcock off, the residual fuel in the bowl goes thru this.

Gas tanks are a totally different thing. Keep them filled, or completely empty. Atmospheric changes over long storage (pressure, temp, humidity), the air moves in and out of the tank, moisture condenses, water in the tank. Fuel turns rancid from replenished oxygen. Really bad with our hydroscopic gasohol. A full tank doesn't 'breathe' as much...

 

[TwoManyXS1Bs]

*Whew*, busy forum tonite.

This discussion about near term vs long term effects reminds me of the "boiling frog" story.

Bob, check your land deed and survey. If it has lat/lon coordinates dated prior to 1929, there may be a problem. As much as we like to think that our acreage stays put, turns out that the USGS had to recompute and republish the coordinate system due to magnetic pole shift and plate tectonics. I think some places on the west coast moved a couple thousand feet...

 

[mrtwowheel]

I own my neighbor's house? I'm going into the U-Haul business.

 

[xjwmx]

xjwmx: in that last scenario all is fine Until you park the bike and leave the fuel on..... then you have an overflow condition ...Correct ?
.....
Bob......

Yes that is the point. When the bike is parked no gas is being burned so no leak is permissible. Pretty demanding.

 

[Bob Kelly III]

ACK ! and my next door neighbor there is backed up to BLM land ( bureau of land management)
but I know for a fact the property was divided when I was a kid in Grammar school ! so it's safe !
the place I am in NOW..... I dunno but it's a city lot , what are they going to do kick me out of my home because some idiot decided to change measurements ? that will never happen ! I promise you ! , .....doesn't mean they won't try though !
that sort of thing is a perfect example of people with too much time on their hands ! time to buy more ammo I guess !
.....
Bob.......

 

[fredintoon]

- - - that sort of thing is a perfect example of people with too much time on their hands ! time to buy more ammo I guess ! - - -

Hi Bob,
my buddy Henryk was born in Poland and raised in Chicago and regards life as an adversarial process.
He told me one time:-
" The pessimists are buying land and learning to speak Russian."
I told him:-
"No, those are the optimists. The pessimists are buying ammunition and learning to speak Chinese."
But about the carb float valves:-
What they are supposed to do is control the carbs gas level when the engine is running and shut the gas off when it stops.
But you can't trust the bastards.
Which is why the gas tank has a petcock.

 

[mrtwowheel]

We need another survey,

Do you trust your float valves more than you trust your petcock, or....................?

Scott

 

[Bob Kelly III]

LOL Thanks Fredintoon ! I needed that ! either way we're screwed ! but what he heck ! it's Just life ...Right ? ya can't take it with ya anyway !
hehehehhehe !
....Scott: Ok I agree it boils down to that !...... so start another survey and lets see what it kicks up ! ......
.....
Bob.......
I don't trust the floats to work perfectly all the time so I use the petcock ! .... and as far as I can tell the forward position on that dern vacuum petcock is off..... cuz no gas comes out'a it ! so that's where it goes when I get off. HA !

.....
Bob........