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Discussion Starter #1
Had a fuel leak - ended up pulling the sender unit out. The fuel gauge needle has always jumped around from zero up to where it should be. There's a brassy metal strip about 1/4" wide from the potentiometer up to inside of the flange that bolts to the tank where it attaches to an insulated bolt that the wire to the gauge connects to. The metal strip had broken and was making and breaking contact with the bolt, hence the needle jumping around.

Soooo.....an electical circuit was constantly making and breaking, which must surely have resulted in a spark inside the tank!!??!? Near the top, above the level of the fuel, where there would have been air and petrol vapour. So how come it didn't just go bang? As the tank is right behind the back of the seat, I reckon I'd have noticed! So was I lucky or am I missing something?
 

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Probably all fume & very little oxygen , or the broken bits too dull to make a contact. Still damned lucky though!
 

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Discussion Starter #4
Spose I'd better fix it then! Seriously though, the truck's only had a gerry can of petrol at a time and it's been started and left running while I try and stop it leaking vital fluids all over the drive lots and lots of times. Surely I'm not that lucky? There's bound to have been plenty of air in there. Anyone know what ratio of air to fuel is needed for ignition and normal atmospheric pressure?
 

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You generally find that the air/furl ratio in the tank is too high, the "atmosphere" in the tank will be above the UEL (upper explosive limit) and therefore there will not be enough oxygen to sustain a fire, hence no bang. On the other hand, if there is too much oxygen, there is not enough fuel to feed the fire, this will be below the LEL (Lower Explosive Limit)The "explosive" band is roughly between 8 - 15% fuel to air ratio (God, I'm sad)
Also, fuel is conductive ( the fuel companies add additioves) as fuel generally tends to build up a static charge when tanks are charged / discharged (for refinery and transportation purposes) so they need to be conductive to disipate the charge. (Am I boring you yet?)
finally, your fuel gauge will probably be of the potentiometer type, only Eddy current, V low voltage, just measure the difference across the device.
And remember, it's only "explosive" if the mixture is "contained", it'll just burn really unimpressively in open air.

I think I need to get out more (time for a career change too, been in fuel testing and on fuel farms for far too long......... :-$

Lol
 

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You'd crap yourself if you thought too long about the in-tank pumps. They have electric motors running submersed in the fuel.
Mart.
 

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You'd crap yourself if thought thought about a lot of the things you use everyday, washing machines and kettles and electric showers, hmmm, electric and water, now there's an accident waiting to happen.....
:)
 

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You generally find that the air/furl ratio in the tank is too high, the "atmosphere" in the tank will be above the UEL (upper explosive limit) and therefore there will not be enough oxygen to sustain a fire, hence no bang. On the other hand, if there is too much oxygen, there is not enough fuel to feed the fire, this will be below the LEL (Lower Explosive Limit)The "explosive" band is roughly between 8 - 15% fuel to air ratio (God, I'm sad)
Also, fuel is conductive ( the fuel companies add additioves) as fuel generally tends to build up a static charge when tanks are charged / discharged (for refinery and transportation purposes) so they need to be conductive to disipate the charge. (Am I boring you yet?)
finally, your fuel gauge will probably be of the potentiometer type, only Eddy current, V low voltage, just measure the difference across the device.
And remember, it's only "explosive" if the mixture is "contained", it'll just burn really unimpressively in open air.

I think I need to get out more (time for a career change too, been in fuel testing and on fuel farms for far too long......... :-$

Lol
If that was boring, then I am boring too, because I found it quite interesting!
 

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Just had a fuel hose split on the 34 whilst driving it. I smelt fuel but had just filled up and it always smells a bit on rh bends for the first few miles so wasn't worried. Holley comp pump so high pressure over the right hand exhaust manifold for at least 4 miles. Pulled into my garage adjacent to my brother working on his Panhead. He was screaming at me to get it back out as he didn't want his bike destroyed!
Still not sure why that didn't go up.:eek:
 

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Because the fuel squirting onto the hot manifold was below the auto ignition temperature and petrol lacks the abilty to sustain self combustion unlike diesel which is a totally different fuel. It's all to do with the free / lighter hydrocarbons escaping from the surface, (that's why petrol goes off and when left standing you get that rusty coloured sludge in you tank / carbs, it's the petrol without the lighter / free hydrocarbon molecules innit) even I'm boring myself now.
Also, due to it's below the auto ignition temp, you need a spark or naked flame to ignite the fuel.
That's why petrol engines have spark plugs and deisel's don't.

You can quite easily see why fuels are made conductive for the reason in the video.
Only the fuel vapour escaping just around the nozzle was between the LEL and UEL, hence for the localised flare.
Apologies for boring you all again

:)
 

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Not boring...think I get that! Hence why all those times we've all adjusted Holley float levels on hot engines and got away with it!
 

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That's about it.
The heavier the fuel oil (and viscous) the more heavier mo;ecules it has, so they do not burn as fiercly but the burn for longer and more progressively (lorries have bags of torque as the fuel is still burning as the piston is going down) and the lighter . less viscous the fuel, the more explosive the burn (natural gas for example Gas turbines).
I'm glad you found it interesting.

Now back to the ironing (and I'm not joking!!!!!!!!!) just managed to get baby to sleep and missus is doing college work, how the other half live eh?
(not the career change I was thinking of....)

LOL

:p
 

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Re the light and heavy ends. I had this explained to me years ago by someone who worked for Esso Refinery ( about 30 miles away from here) . I've been able to impress a few people who have tried in vain to start their car after it's been stood around for 6 months or so just by dribbling abit of fresh fuel into the carb to start a reaction.

I fell for the " but's it got fuel and a spark it must start " myself in the past until educated.
 

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Glad to be of service (even if you suffer from insomnia, I hope you slept after reading that) it nice to draw on the generally unuseable knowledge now and then.
Cool :)
 

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Discussion Starter #17
Well, I'm reassured. Thanks for the technical input. I was ready to spend some hard-earned on a new tank to go under the pickup bed.

Glad to be of service (even if you suffer from insomnia, I hope you slept after reading that) it nice to draw on the generally unuseable knowledge now and then.
Cool :)
 

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I'm assuming that you're just going to replace the sender and use what you already have,, then all will be well and your mind will be at ease (as will everyone elses who you try and explain this too)
 

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Discussion Starter #19
Kind of - I soldered the strip back together and the needles not jumping about now. I'll probably still go for a new tank at some point and put it under the pickup bed at the back as I suspect weight distribution isn't quite the perfect 50/50!

Good subject to discuss with passengers though.
 
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