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I assume the floats can resist ~14psi since that is atmospheric, but does anyone know if that is true? I want to put a low-pressure fuel pump into a new tank design that will sit under my seat but I need to clarify what pressures the carbs floats can handle.
My guess is that I'll have to wire up some kind of load sensor into the pump wiring (probably just a PIC setup) so that when the fuel inlet is closed in the carb the pump won't burn itself out. Wolfe if you have any input on the electrical side of things that would be greatly appreciated (O Great Electron Guru).
First off I do not think that atmospheric pressure would relate to this question as that pressure would be on both sides of the float valve, unless you have a perfect vacuum in the float bowl of the carb. Then I think you would have other problems!
I would say you need a low pressure pump that can flow the amount of fuel your engine needs at full throttle plus a little for safety margin. That side this pump or one similar to it should work:
Not sure if there are NAPA dealers in BC? Listed on that page is the make and manufactures Part Number 60304 so you might be able to locate one that way.
This pump is rated at 1 to 2 psi and 7 gallons an hour. That is about as low a pressure you will find and if your XS400 is using more than 7 gallons an hour you are riding it way too hard!
This pump style will not burn out if dead headed, but I would if possible install an oil pressure switch to turn it off when motor is not running or make sure to wire it so that it can't be energized with key or kill switch in the off positions. May want to use a relay in the system to do that.
Interesting question. I can only speculate on a few points.
kshansen beat me to this one: The atmospheric pressure can be ignored as it is present in the fuel tank and float bowl.
A little bit of math and estimating and it looks like the float needle valve is probably only resisting .3psi of fuel pressure from the gravity fed system.
You're probably going to want the lowest pressure pump you can find.
I know that in automotive carbs, the floats can keep pressure on the needle/seats up to around 6 psi. Don't know about our tiny carbs though. Either way, you'll want to run a fuel pressure regulator to keep it steady. Grab a decent one and start low and see what works. You'll know the pressure is too high when fuel won't stop poring out of the carbs
And yes, fuel pumps are designed to be run constantly, even if they aren't flowing. As long as it has fluid in it, it won't burn up. At least that's how in tank pumps work.
doh, of course atmospheric pressure is the same on both sides. I guess I was trying to wrap my head around the fluid forces in the gravity feed but I didn't bother to apply my basic fluid dynamics knowledge... serves me right. I'll do my own calcs and see what I come up with to get a better idea of the operating psi.
Pressure regulators that I can find go down to around the 1psi mark so if I need less I may be hooped. I'm thinking a may do a method I found called the "magic can" that would have the fuel pump running into a second cylinder with one outlet going to the carbs and the other on top feeding back to the tank. If the floats close then the fuel returns to the tank. I guess it's a "gravity simulator" of sorts.
As for wiring, I'll be doing up my harness custom so I can work any relays or switches in easily.
Hobbs switch could regulate a low psi pump, if you could determine a cutoff pressure when the floats close, I.e. When floats are open nominal pressure is 2psi when they close pressure is 3psi. Hobbs switch rated for 3psi cuts pump. But there would have to be a pressure differential. Blow into your fuel line it is super easy to over run the floats and I don't think I could produce over 3psi via lungs if I tried.
Here's some smart talk.. http://www.ashonbikes.com/forum/fuel-pump-carbd-bike-why
doh, of course atmospheric pressure is the same on both sides. I guess I was trying to wrap my head around the fluid forces in the gravity feed but I didn't bother to apply my basic fluid dynamics knowledge... serves me right. I'll do my own calcs and see what I come up with to get a better idea of the operating psi.
