SOHC.co.uk Forums > CB750
Head Bolts
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 ;D
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 ;D
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):
--- 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 ---
Good point Moorey about the debris issue.
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