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Old 03-Mar-2006, 21:29
851neil 851neil is offline
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jgriff, I don't understand your explanation / reasoning.

The return side is only dealing with unused fuel from the injectors, these are pumping fuel back under pressure and this is fed thru the pipework and connectors into the degasser, and then straight back into the tank & not to the pump.

It's doing this as there are only certain throttle openings and revs where just about all of the fuel supplied is used by the injectors, at points below this only one injector is firing but the system is seeing fuel at the same pressure as when at high revs, hence the return of fuel under pressure.
On single injector motors the same symptom occurs thru the rev range, check out the injection timing charts in the falloon books, or the moto one performance index.

I know that the degasser was fitted as Duc had big problems with tank pressurisation which starved the motor of fuel, even when a tank breather was fitted - I think it starved the motor of fuel as the back pressure was greater than the pump pressure - hence the degasser and also the 'non-reversion valve', the valve halted back pressure and allowed one way flow only and a motor not starved of fuel.

This is the only logical reason I can come up with for it being there on the works bikes, more fuel demand, bigger pressures, bigger heat soak etc...led to tank over pressure
cheers

Neil
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Old 04-Mar-2006, 04:51
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jgriff jgriff is offline
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Yeah strange isn't, whats worse is I can't find where i read it again. I always just thought it was to stop the fuel coming out when you disconnected the tank, as per the manual.
Ok the fuel system. The pump puts out somewhere between 3-5 bar constantly. it draws from the bottom of the tank. Fuel leaves the tank and feeds the injectors. Next in the circuit is the pressure regulator, the pressure regulator bleeds off any pressure in excess of 3 bar to the return line. (Aftermarket can be set to 4.5. This gives more fuel per injector cycle, and hence more go. FIM). 3 bar is 44 ish psi. So the the return pressure can be any where from 0 to 2 bar depending on injector demand. Max pump pressure 5 bar minus system pressure 3 bar.
The degaser just removes any air that may be in the circuit, from cavitation or maintence.(In avaiation we use to call them swirl cans.)
The tank needs a breather to allow for over and under presurisation. Use the fuel you get negative pressure and pump will starve for fuel, and cavitate. Add heat and tank will have positive pressure. Tank will get fat and burst. A little positive pressure means the pump doesn't have to work so hard. So you would think that this is a good thing. But it will cause more pressure on the back surface of the pressure regulator diaphram, thereby effectively increasing the regulators spring force and raising regualted pressure above 3 bar.
So our non return valve now has two jobs. Stop fuel ****ing out all over the place when you take the tank off. (Fuel pump has its own non return valve), and stops an increase in fuel pressure regulation in the event of a fuel tank increase in pressure, by maintaining a set return line pressure using the opening spring force in the Non return valve.
How about that then? Sound better.

[Edited on 4-3-2006 by jgriff]

[Edited on 4-3-2006 by jgriff]
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