Steve, thanks for that, you are quite correct of course, but in this case the problem is definitely due to petrol getting in the float. When taking the top off the float chamber after it flooded the float was almost submerged but after emptying it there was about 1/4 inch showing above fuel level and thereafter the needle valve operated as it should
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One mans junk is another mans treasure
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Suggests your soldered joint was not up to scratch,If the joint being soldered is dirty you wont get a decent seal even though it looks good.
How did you do it?
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Steve
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Suggests your soldered joint was not up to scratch,If the joint being soldered is dirty you wont get a decent seal even though it looks good.
Touche..I will suffer this criticism even though I have soldered countless copper pipe joints without any leaks, -:)...however a poor solder repair patch does not explain why it intially filled up two years ago.
It's all acedemic as first thing tomorrow I will order a new plastic one, however I dislike mysteries and want to find the hole or fault that is causing this....is there any liklihood that the original soldered seams have become porous due to Auntie being forced to consume this new-fangled unleaded petrol?
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One mans junk is another mans treasure
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On the subject of the new nylon float (new brass ones no longer being available) it is getting late and I can't get my head round whether or not fitting this will alter the level of fuel in the float chamber.
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Well, I'm sure it will be just as good as accurate as the metal one you have with all of its solder "ballast"!! For small changes in float mass or volume, the adjustment of the jet height via the mixture adjustment will compensate.
Be careful if you are weakening the mixture, because these carburettors do not spring load the needle, and it is possible to make a previously free sliding piston bind up if the centreing of the jet is not spot on.
Number_Cruncher
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One way to find the source of the leak is to immerse the float in hot water. The heat makes the air inside the float expand and leak out through any pinholes. It's the same principle as pumping up a leaky bicycle tube, which makes the puncture much more obvious.
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You might find the following website useful and the owner is very helpful. www.su-carbs.co.uk. Andrew Turner 01572767665. No connection, other than as a near neighbour, who sees him doing loads of work on SU and Stromberg equipped cars and bikes!
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The volume of the float is immaterial,it will stop the flow of fuel when it shuts the needle valve.
My theory is that you have a very small crack.When tested in hot water,to expand the air,it also expands the brass and closes the crack.When exposed to cold fuel from the tank it contracts the metal and opens the crack.Also cooling the air within the float will draw the petrol in.
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Thanks sierraman, that is a very plausible theory, though not as romantic as my notion that the sulpher in unleaded petrol might be making the solder porous :)
I will now stop pondering and get on the phone for a new plastic one. As Number Cruncher suggested, hopefully the mixture adjustment available on the jet nut will compensate for any variation in the fuel level as a result of fitting this.
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One mans junk is another mans treasure
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>>The volume of the float is immaterial,it will stop the flow of fuel when it shuts the needle valve.>>
Yes, it is the buoyancy not the volume that is important so the plastic one will probably be smaller though just as buoyant, therefore assuming it is designed for the particular carb in question a plastic one should cut the fuel off at the same level as the copper one did.
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Buoyancy and volume are linked quite intimately.
In equilibrium, (for a float on a pivot, include the necessary moment arm length)
The fuel pressure multiplied by the valve area, plus the weight of the needle valve, plus the weight of the float are balanced by the buoyancy force, which is simply the mass of fuel displaced by the float. The weight of fuel displaced is, of course, linked to the size and shape of the float.
Interestingly, the stiffness of the float in the fuel can also be varied by changing the shape.
Number_Cruncher
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I seem to remember from my school physics classes that a floating object will displace it's own weight of liquid regardless of volume above or below the surface.
I guess this means that a plastic float could be moulded to protrude the same height above the fuel level as the brass one did when it was in good fettle. Sufficient buoyancy is equally important to hold the needle valve shut against pump pressure and engine vibration, so it has to be large enough.
Quite a challenge for the man who had to design this apparently simple replacement plastic bit to fit in a existing housing.
£15 doesn't seem so extortionate after all.
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One mans junk is another mans treasure
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I seem to remember from my school physics classes that a floating object will displace it's own weight of liquid regardless of volume above or below the surface.
Interesting point, the amount that is above or below the surface depends on the density (or weight v volume) of the floating object relative to that of the liquid it is floating in, i.e. the tip of the iceberg scenario is because the ice has a very similar density to the water it is floating in.
Buoyancy and volume are linked quite intimately.
Buoyancy and density are linked quite intimately. It is possible to have two objects of similar volume though of a very different density, one may have a greater density than the applicable liquid and will accordingly sink, the other may have a lesser density than the liquid and will float.
In the carb float scenario the weight of the material that the float is made from effects the float's density, a smaller plastic float can do the same job as a larger metal one because it has a lesser density and accordingly a lesser proportion of its displaced volume is being used to support its own weight.
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"a floating object will displace it's own weight of liquid regardless of volume above or below the surface."
No - its volume below the surface is directly related to how much liquid it can displace. If it cannot displace its own weight, it will sink. If it can, it will float
"the tip of the iceberg scenario is because the ice has a very similar density to the water it is floating in"
It also depends on the shape of the iceberg above and below the water. You could in theory have a very tall hollow ice "mast" projecting hundreds of feet above the surface.
Also if you built a ship out of ice, it could, like one of ferro-concrete, float mostly above water.
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"a floating object will displace it's own weight of liquid regardless of volume above or below the surface." >>
No - its volume below the surface is directly related to how much liquid it can displace. If it cannot displace its own weight, it will sink. If it can, it will float
The first statement is infact correct.
An object placed in a liquid will always displace its own weight, the amount of the object that is above or below the surface of the liquid depends on the density of the object, the denser the object the more that will be below the surface, if the object is denser than the liquid it will still displace its own weight and it will sink in doing so.
Of course the shape of the object can determine how far above the liquid the object protrudes however two objects that are different in shape though of the same density will have the same proportion of their mass above and below the liquid surface.
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<< if the object is denser than the liquid it will still displace its own weight and it will sink in doing so.>>
I can't see that this is true. The object will displace its own volume of liquid, but not its own weight. If you take 2 equal size cubes of, say steel and uranium, then both will displace the same volume of liquid, but they won't weigh the same, only the displaced weight of liquid will be the same in both cases.
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<< if the object is denser than the liquid it will still displace its own weight and it will sink in doing so.>> I can't see that this is true. The object will displace its own volume of liquid, but not its own weight. If you take 2 equal size cubes of, say steel and uranium, then both will displace the same volume of liquid, but they won't weigh the same, only the displaced weight of liquid will be the same in both cases.
Yes you are of course correct, the point I was making is that in a scale of density there is a precise point where the density of the object becomes greater than that of the liquid so the object will sink and will displace its weight in doing so, of course if you continue along the scale and place ever denser objects in the liquid they will simply displace a weight in liquid equal to their respective volumes.
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But are we taking into consideration the effects of surface tension on the object as it is about to sink?
...quickly leaving the Back Room :)
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One mans junk is another mans treasure
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That may be your problem, chill out, lose the surface tension, and maybe your float won't sink ---.
Have you tried drying it out on the outside, with petrol still inside it and looking for the weep if you turn it over in your hand? May be easier to see than air bubbles. Do it carefully of course, we don't want any backroomers unintentionally taking off, do we?
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Not going to matter what surface tension is,at the end of the day float is not working,if float is doing its job and sealed it will do its job,if however the valve is not.
No float in the world is going to prevent float from going under,but will remain sealed.
so kinda says to me,you either have bad floats,or valve is not sealing!.
Which I think is the case
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Steve
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<< if the object is denser than the liquid it will still displace its own weight and it will sink in doing so.>>
Steel is denser than water,ships have been made from steel for some time now,they only sink when water gets inside,a bit like this float.If the object is solid,has density equal to that of water then the volume displaced will weigh the same.
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Steel is denser than water,ships have been made from steel for some time now,they only sink when water gets inside,a bit like this float.If the object is solid,has density equal to that of water then the volume displaced will weigh the same.
When referring to a hollow object such as a float or a ship it is not the only density of the material that forms the skin / hull that is relevant, rather it is the combined density of the skin / hull material and the contents (air, cargo etc).
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