Head Bolts

[Oddjob]

Thank you, ordered the Sealey version, to hook on the rafters. 👍

You might need to watch the header tank height as too high might cause the float valves to leak.

That I feel is a fallacy Ted, if it seals it seals, look at the video of that wreck that Johnny started up the other day, tank is very high in the rafters and that didn't leak, and they were PD carbs as well, known for causing problems. Sure pressure can cause problems but what weighs more, 3 gallons a few inches above or 1 pint a few feet above?

[McCabe-Thiele (Ted)]

Thank you, ordered the Sealey version, to hook on the rafters. 👍

You might need to watch the header tank height as too high might cause the float valves to leak.

That I feel is a fallacy Ted, if it seals it seals, look at the video of that wreck that Johnny started up the other day, tank is very high in the rafters and that didn't leak, and they were PD carbs as well, known for causing problems. Sure pressure can cause problems but what weighs more, 3 gallons a few inches above or 1 pint a few feet above?

The pressure is not affected by the fact that a tank holds 3 gallons - it's all about the pressure per square inch so a water tower at say 66 ft above ground level will have a pressure of about 30 psi to a tap at the bottom. A pint of water in bottle at 66 ft will have the same pressure of about 30 psi.

IIRC petrol is heavier than water but lets says it's the same weight per gallon as water. The pressure from the petrol tank if its a foot from top of fuel in tank to the carburetta will be what just under 0.45 psi. If the temporary tank is held at a height of 3 ft you are increasing the pressure to 1.35 psi so there must be a greater risk of a leak at three times the pressure.

That's my logic I'm open to being told where I am mistaken.

[Oddjob]

My point is Ted that we seem to blame the height of the tank NOT the fact the float seat is leaking already. Johnnys tank was pretty high yet no leaks, yet we blame the height of the tank when it's just on the bars, it's not really the height that's causing the leakage, it's the fact the seal wasn't that good in the first place. Personally I've never seen a carb leak when a remote tank is used that wasn't already leaking when connected to the petrol tank.

The fact that Johhnys didn't leak at that height (and TBH I've done the same myself, hanging the tank really high) means the float seat can handle that amount of pressure so long as it's in good condition, we blame the height of the tank for leaks when we should really be blaming a poor seat instead.

[K2-K6]

It doesn't change much at all.

 1 ltr of fuel hung on the handlebars is exactly the same weight as it is hanging on the ceiling, just the volume of the pipe (minimal at that bore) is added to the total weight. If you had it on the bars with tube running horizontal to the carbs, then lifted it up high, you just add that tube volume in weight.

Water towers are about distribution over distance to supply surrounding areas I believe. The volume has to be above all of the outlet else the pressure will not be enacted at the outlet when you open that tap if its not reliably below the head tank.

[McCabe-Thiele (Ted)]

I agree with Bryan that if the float needle seal is good in the first place then some extra pressure might be withheld depending on the design of the valve & how much pressure it can resist but there is a point where the  pressure due to the height will be sufficient to open the float valve. Old cars with mechanical fuel pumps typically have a feed pressure of 4 psi so the float valves are designed to withstand this pressure - I have no idea what the equivalent pressure is of a Honda gravity fed float valve - might be it's good to 3-4 psi.

From my days of studying fluid dynamics (yep a long time ago 1971) the lecturer came up with the analogy that fluid pressure is in many ways equivalent to voltage in electronics. The higher the voltage/pressure the greater the ability to overcome resistance.

It's all about the pressure if it's imperial its pounds per square inch so within some limits depending on the friction in the pipe a doubling in pipe diameter does not change the pressure only the flow rate ability at any given height.

[Martin6]

I'm with Ted here. The greater the height, the greater the potential energy, which is converted to pressure on the float needle. There will come a point that the pressure is sufficient that the force is greater than the force exerted by the flotation of the floats. At that point, the float needle overcomes the floats and petrol rushes in.

I don't have the maths to calculate the force required, (clockwise moments = anticlockwise, etc.) but my rafters aren't particularly high and my rope is longer than my spare fuel line   

[Oddjob]

And yet I have a VT1100C which has carbs and a standard fuel valve shutoff like the SOHC and it's got a fuel pump, I'd imagine the pressure that produces far exceeds any gravity feed pressure. Doesn't leak either, I tested the pump last year to see if it was working, it shot water 6ft across the garage, that's pressure.

My point being that a remote tank is pretty much never going to exceed the float valves shutoff systems limits unless you're going to extremes like hanging it off the chimney 

All that it may do is expose the inherent weakness in the system, a lot of people change these for stuff like Keyster and they ain't anywhere near as good as the original brassware.   

[Nurse Julie]

P = pgh
Something I had to learn many years ago for the delivery of intravenous fluids to patients in intensive care wards, before calibrated fluid delivery pumps arrived on the scene.

[Moorey]

     

     Typically mc fuel pumps delivers 3psi.  Float valves are generally overcome at approx 4.5 / 5psi.  1ft head of water gives 0.433psi do the maths.
     One thing many don't think of is that in use float valves open very tiny amounts so the smallest of debris is likely to stay in the valve and never
     be flushed through unless the carb is drained or stripped and cleaned properly.
     Anyone wanting to check simply put a airline with a pressure reg on a upside down carb on the carb inlet and see at what point the valve leaks
     air when using leak detector spray.
     

[McCabe-Thiele (Ted)]

     

     Typically mc fuel pumps delivers 3psi.  Float valves are generally overcome at approx 4.5 / 5psi.  1ft head of water gives 0.433psi do the maths.
     One thing many don't think of is that in use float valves open very tiny amounts so the smallest of debris is likely to stay in the valve and never
     be flushed through unless the carb is drained or stripped and cleaned properly.
     Anyone wanting to check simply put a airline with a pressure reg on a upside down carb on the carb inlet and see at what point the valve leaks
     air when using leak detector spray.
   

Good point Moorey about the debris issue.

[Oddjob]

Exactly what I was trying to say Ted, we blame the height of the remote fuel container instead of looking for the reason it’s leaking, reasons like debris under the seat or poor manufacture of the float seat etc, the higher position of the remote container just makes the existing problem worse. A good seal isn’t bothered by the raised height.

[K2-K6]

     

     Typically mc fuel pumps delivers 3psi.  Float valves are generally overcome at approx 4.5 / 5psi.  1ft head of water gives 0.433psi do the maths.
     One thing many don't think of is that in use float valves open very tiny amounts so the smallest of debris is likely to stay in the valve and never
     be flushed through unless the carb is drained or stripped and cleaned properly.
     Anyone wanting to check simply put a airline with a pressure reg on a upside down carb on the carb inlet and see at what point the valve leaks
     air when using leak detector spray.
   

Isn't that accumulated figure 0.433 psi going downward under sea level ? Usually at one atmosphere for each 10.06 mtrs (33ft) effectively the accumulation of the ocean above you.

In air, and by lifting 1 ltr, then its nothing like that surely.

I'd doubt much difference between a tank say at 20 ltr sitting just above the carbs, in comparison to 1 ltr at 6 ft higher. 

[taysidedragon]

I hang my temporary tank on the bars because otherwise you have to extend the fuel line to reach the rafters.
Why use 6ft when you can just use 2ft? 🙂

[McCabe-Thiele (Ted)]

     

     Typically mc fuel pumps delivers 3psi.  Float valves are generally overcome at approx 4.5 / 5psi.  1ft head of water gives 0.433psi do the maths.
     One thing many don't think of is that in use float valves open very tiny amounts so the smallest of debris is likely to stay in the valve and never
     be flushed through unless the carb is drained or stripped and cleaned properly.
     Anyone wanting to check simply put a airline with a pressure reg on a upside down carb on the carb inlet and see at what point the valve leaks
     air when using leak detector spray.
   

Isn't that accumulated figure 0.433 psi going downward under sea level ? Usually at one atmosphere for each 10.06 mtrs (33ft) effectively the accumulation of the ocean above you.

In air, and by lifting 1 ltr, then its nothing like that surely.

I'd doubt much difference between a tank say at 20 ltr sitting just above the carbs, in comparison to 1 ltr at 6 ft higher.

The volume of petrol in either tank is not the issue here that would only alter the flow level if both tanks were of the same depth.

My previous sums were out by a factor of two as a 66 ft water tower would represent 2 Atmosphers about  60 psi not 30 psi - so a foot of water is around 0.9 psi the total volume of fuel in either the bike tank or the temporary one is irrelevant. So at 6 ft above the existing petrol tank of say 1-ft the pressure would rise from 0.9 psi with an increase of 6 x 0.9 = 5.4 making the total pressure at effectively 0.9 + 5.4 = 6.3 psi. This does not take into account the difference in density between water & petrol , all pressures are based on sea level.

[K2-K6]

It's making me intrigued as to what is correct, not because it's in any way practical.....for reference, I use a plastic syringe about 100ml stuck into the fuel feed pipe and secured to frame by elastic band when carb balancing. Same one has been in my tools since the seventies 

Can we apply the "atmosphere" calculation to a static head of fluid though ? The example of going down intomthe sea doesn't seem to apply to me as you are effectively within the fluid that is supported on the seabed. Each movement downward gives you a layer now above you that's compressive. The total gain taking into account that increases in mass to give the typical calculation figure.

In water tower or the high level fuel feed, the mass is supported through a route down to the ground and so doesn't enact a force on the outlet.

The column weight is what we could measure at base level, this then depending on bore size (capacity in vertical now unsupported by the structure) with a column of 60 ltrs that would be 60 kilos in water into whatever sized outlet we have.

Oil derived fuels are ordinarily less dense than water, but not by much to affect our consideration here.

It looks more like volume of pipe above float valve (possibly 100 ml ) when on ceiling, approx 100 grams resisted by the float force through leverage on the entry valve.