P4 SU carburettor flooding puzzle - Steptoe
As the more *mature* members of the Back Room were very helpful with a solution for the last conundrum Auntie set me, that of the dirty solenoid connection, I wondered if I might lay the latest problem on the collective table.

Auntie has an easy life these days, only venturing out to occasional rallies, but about two years ago she was reluctant to return home from one, due to severe flooding of the H-type SU carburettor. I quickly determined that this was due to the float (original soldered brass type) having sunk to the bottom of the chamber. Try as I might, I couldn't get the petrol out of the float until I punctured it. I then emptied the float, screwed a self tapper in the hole I had made, and drove home. After soldering up the hole I had made, I tried to find how the petrol originally got into the float (immersing in hot water etc.) without any success. Eventually I gave up, put everything back together, and added 'ignition on; wait for SU pump to stop ticking; start engine' to my pre-flight check list.

This morning, the pump didn't stop (incidentally it was perfectly OK yesterday). Mercifully as I was still at home, the procedure was straightforward enough; try to get petrol out of float, fail to do so, drill through soldered patch, shake petrol out, insert self tapper & rubber washer, immerse float in hot water to try to find original leak, fail to do so, put everything back together and off to the rally, only 20 minutes behind schedule.

The solution of course is a no-brainer; purchase a nice new reliable nylon float. However the puzzle remains; how did the petrol get into the float? On each occasion the time interval between the float having enough buoyancy to close the needle valve and sinking below the critical point was only a matter of hours, which would suggest that the entry point for the fuel should be large enough to find. However not only was the puncture impossible to find but it has taken two years for the float to refill sufficiently to sink, which would suggest a minute seepage.

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. Presumably it will be of different density to the original brass version; will it therefore displace more or less fuel in the chamber thus altering the level or is it simply a matter of the volume of the float setting the level in the chamber, this presuming new and old are of similar dimensions. Aunties liquid intake is so great in any case that it is fairly important to get the mixture just right. Thanks in advance for any views on this.

----------------------------------------------

One mans junk is another mans treasure
P4 SU carburettor flooding puzzle - Civic8
Float operates a valve,this valve determines amount of petrol going into float chamber,if float or valve fail it gets overfuelled.

Suggest checking the float valve is working correctly, as if it isnt float will be useless
--
Steve
P4 SU carburettor flooding puzzle - Steptoe
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
----------------------------------------------

One mans junk is another mans treasure
P4 SU carburettor flooding puzzle - Civic8
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?
--
Steve
P4 SU carburettor flooding puzzle - Steptoe
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?

----------------------------------------------

One mans junk is another mans treasure
P4 SU carburettor flooding puzzle - Number_Cruncher
-----8<--------

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.

-----8<--------

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
P4 SU carburettor flooding puzzle - Pete M
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.
P4 SU carburettor flooding puzzle - Armitage Shanks {p}
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!
P4 SU carburettor flooding puzzle - sierraman
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.
P4 SU carburettor flooding puzzle - Steptoe
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.
----------------------------------------------

One mans junk is another mans treasure
P4 SU carburettor flooding puzzle - cheddar
>>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.
P4 SU carburettor flooding puzzle - Number_Cruncher
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

P4 SU carburettor flooding puzzle - Steptoe
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.






----------------------------------------------

One mans junk is another mans treasure
P4 SU carburettor flooding puzzle - cheddar
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.
P4 SU carburettor flooding puzzle - Cliff Pope
"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.
P4 SU carburettor flooding puzzle - cheddar
"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.

P4 SU carburettor flooding puzzle - mjm
<< 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.
P4 SU carburettor flooding puzzle - cheddar
<< 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.
P4 SU carburettor flooding puzzle - Steptoe
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 :)
----------------------------------------------

One mans junk is another mans treasure
P4 SU carburettor flooding puzzle - mjm
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?
P4 SU carburettor flooding puzzle - Civic8
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
--
Steve
P4 SU carburettor flooding puzzle - sierraman

<< 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.
P4 SU carburettor flooding puzzle - cheddar
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).
P4 SU carburettor flooding puzzle - Victorbox
Are you sure the brass float is not available for your carb? Over at Burlen Services www.burlen.co.uk/ they list quite a few brass floats for H-Type SU carbs. Not being a P4 expert I can't of course comment on whether they are what you need. Burlen may even have a good used spare.
P4 SU carburettor flooding puzzle - Cliff Pope
It's lovely to see the way this thread keeps returning to the original point - some of our replies actually refer to carburettor floats!

The point about the weight displacement thing is that it only applies to a FLOATING object. Cubes of steel and uranium will both sink, because neither (if solid)can displace its weight, because the water is not dense enough. Substitute mercury, and perhaps the steel might float? (Not sure without looking it up)
P4 SU carburettor flooding puzzle - Steptoe
After having stirred this subject up, so to speak, and set the BR the nylon/brass conundrum, I am somewhat embarrassed to have to confess that the new float which arrived in the post today is indeed a brand spanking new soldered brass one, as Victorbox suggested it might be.

I can't remember what gave me the idea the new one would be of nylon construction, but can only assume it was when I initially enquired about a replacement item when the flooding happened two years ago.

Thanks for everyone's help, sorry about the red herring, but hopefully the discussion about displacement has added to the sum total of our knowledge.

Once I have the old one out I will examine it more closely, if the elusive leak turns out to be anything interesting I will add a footnote on this thread.



----------------------------------------------

One mans junk is another mans treasure
P4 SU carburettor flooding puzzle - Old Rover
Forgive the thread resurrection.

Just had the same problem on this model of carburettor. The leak takes place through hairline vertical cracks in the outer wall of the float. I tried opened one up , drained the petrol and soldered it but a few days later it was leaking again. No soldering kit available this time so it was pierced with a knife and sealed with milliput. Hopefully this will keep it petrol tight till a new float from Burlen arrives in a few days. I use the vehicle, a 107" land rover with P4 60 cylinder head as a daily runabout and work vehicle.

 

Value my car