Fuel pressure on Carb motors

[CobraEd]

Originally Posted by Jerry Clayton

The inlet valves are NOT OPENED by fuel pressure!!!Gravity causes the float to drop, opening the needle seat.

You are right, . . . . I forgot!!


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[Bobcat]

Let`s go back to basic theory . A fixed displacement pump ( fuel , hydraulic or whatever ) delivers volume , or flow , in gpm , lpm or whatever . A restriction in the circuit creates the pressure , or more precisely , pressure drop across the restriction . In this case , the restriction/orifice is/are the openings in the float needle valve . The pump will deliver a flow of so many gpm across the orifice at a certain pressure drop directly related to the pump flow . Assuming the pump flow is constant and you could vary the orifice , you would see the pressure drop increase or decrease as the orifice size was decreased or increased . The float provides in effect , a back pressure check that varies somewhat the orifice size ... although it is fairly crude .
Why would volume drop as pressure increases ? Pumps are not 100 % efficient and always have some internal leakage ... either through the internal valves ( diaphragm pump ) or past the gears or pistons . The needle vale also isn`t a 100% bubble tight seal when closed .
Also , most fuel pumps have an internal relief valve that limits the max pressure ... as do some piston pumps .


Bob

[priobe]

Originally Posted by Bobcat

Why would volume drop as pressure increases ?

Bob

Ok, take a water hose, turn open the valve you have alot of volume, basically no pressure.

Apply your thumb over the hose outlet ( restriction).

Pressure increases / volume decreases.

[Bobcat]

Pretty much correct , except that a water hose doesn`t work exactly a pump . The hose has no internal bypass ( unless it ruptures ) . If you put a variable cut off valve on the output of the hose with a pressure gauge teed in and close it , you will see whatever the system pressure is . Watch the gauge as you open the valve and you will see the pressure fall off as the flow increases .
In a hydraulic system , if you have enough hp behind the pump , you can push as much flow through the orifice as you want up to the flow capacity of the pump . You just increase the pressure drop ( delta p ) across the orifice . The reason pump flow drops off as pressure increases is because all pumps have internal leakage from clearance between the gears to housing , pistons to sleeves and housing etc and on diaphragm units ( fuel pumps ) , across the check valves . Other components in the system can and do have internal bypass/leakage . All pump circuits have a relief valve somewhere in the outlet ... or you will have a disaster if the output is closed off totally . I`ve seen gear pumps literally explode when the output was blocked .

Bob