SOHC.co.uk Forums > CB750
Head Bolts
Moorey:
--- Quote from: K2-K6 on January 20, 2024, 02:55:49 PM ---
--- Quote from: Moorey on January 20, 2024, 10:04:31 AM ---
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.
--- End quote ---
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.
--- End quote ---
No just as a column of water lift the tank 10 ft and you will have 0.433 psi X 10 = 4.33psi at the pipe end. Volume in the tank is irrelevant.
A quick Google
What is the pressure exerted by a 1 foot column of water equal to?
Using Water as a Guide for Determining Pressure – Math for ...
0.433 psi
The relationship between water, height and pressure is constant. If we were to take a column of water 1 foot high and use a pressure gauge at the bottom to measure the pressure it would read 0.433 psi. This is our constant when dealing with water and it remains consistent as we add more water.
Moorey:
--- Quote from: McCabe-Thiele (Ted) on January 20, 2024, 03:45:49 PM ---
--- Quote from: K2-K6 on January 20, 2024, 02:55:49 PM ---
--- Quote from: Moorey on January 20, 2024, 10:04:31 AM ---
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.
--- End quote ---
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.
--- End quote ---
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.
--- End quote ---
Where do you get 0.9psi from for a column of water 1ft high.
McCabe-Thiele (Ted):
--- Quote from: Moorey on January 20, 2024, 11:01:07 PM ---
--- Quote from: McCabe-Thiele (Ted) on January 20, 2024, 03:45:49 PM ---
--- Quote from: K2-K6 on January 20, 2024, 02:55:49 PM ---
--- Quote from: Moorey on January 20, 2024, 10:04:31 AM ---
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.
--- End quote ---
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.
--- End quote ---
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.
--- End quote ---
Where do you get 0.9psi from for a column of water 1ft high.
--- End quote ---
Actually I am wrong it will be half that as 14.7 psi is equivalent to a water manometer 33 ft in height at sea level.
So 33 ft of water exerts a pressure of approx 14.7 psi, divide 14.7 psi by 33 gives you 14.7/33 = 0.44545 psi per 1 ft column of water.
As petrol weighs around 7.2 lbs per gallon as against water weighing 10 lbs per gallon a one foot column of petrol would exert less pressue it would be 7.2 divided by 10 multiplied by 0.44545 psi i.e 0.3207 psi.
So if a petrol tank has a height of 1 ft between the fuel tank level & the carbs that would be 0.3207 psi exerted by the petrol.
Increasing the height some extra 6 ft up in the rafters from the tank level would be an additional 6 x 0.3207 =1.9242 psi so a new total of 1.9242+0.3207= 2.2449 psi.
My revised figures are only accurate to a couple of decimal places this leads me to the conclusion that Ken, Bryan & others are right that is unlikely to make the float needle leak under such a relatively low pressure.
My warning about hanging the reservoir in the rafters is clearly a load of Gonads it is unlikely to cause a healthy float needle to weep.
Pleased you pulled me up on my maths.
Oddjob:
Ted please don't for one second think we say bunkum just because it's you saying it, I've been thinking about that one for a good while now and realised it didn't fit with my experiences, a lot of what's out there on the web is pure rubbish, having a debate about it can be very helpful for solving whether it's good advice or just plain garbage. You won't be the first to repeat something they've read somewhere thinking it sounded true, you certainly won't be the last.
If nothing else it made people hone up on their maths skills. ;D ;D
McCabe-Thiele (Ted):
For some dim reason Ken I doubled the approximation of 15 psi to 30 psi thinking about gauge pressure & absolute pressure so bunkum applies - I have my Birch Twigs at the ready for some self-flagellation.
The reality of my twice revised figures now explains your decades of working experience to my complete satisfaction.
PS I've got a dose of Insomnia hence my posting at gone 2.00 am
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