SOHC.co.uk Forums > CB750
Head Bolts
Oddjob:
--- Quote from: McCabe-Thiele (Ted) on January 19, 2024, 02:00:20 PM ---
--- Quote from: Martin6 on January 19, 2024, 12:38:25 PM ---Thank you, ordered the Sealey version, to hook on the rafters. 👍
--- End quote ---
You might need to watch the header tank height as too high might cause the float valves to leak.
--- End quote ---
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):
--- Quote from: Oddjob on January 19, 2024, 04:40:57 PM ---
--- Quote from: McCabe-Thiele (Ted) on January 19, 2024, 02:00:20 PM ---
--- Quote from: Martin6 on January 19, 2024, 12:38:25 PM ---Thank you, ordered the Sealey version, to hook on the rafters. 👍
--- End quote ---
You might need to watch the header tank height as too high might cause the float valves to leak.
--- End quote ---
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?
--- End quote ---
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.
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