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Fuel flow location on carbureted engines

FireMedic_2009

Well Known Member
The location of the fuel flow meter is as follows: fuel valve, fuel filter, fuel flow meter, electric fuel pump. The distance between the filter and meter is 2” and the distance between the meter and elec fuel pump is about 5-6” with a 45 deg offset. When I turn ON the fuel pump the gph increases about 6 gal or so. I believe the meter should be between the engine fuel pump and the carburetor, correct?
Is the extra gph when the electric fuel pump is On going out the fuel overflow line from the engine fuel pump? The electric fuel pump doesn’t have a return line like you would have on the electric fuel injection fuel pumps. It’s an automotive Facet fuel pump.
Thanks
 
The location of the fuel flow meter is as follows: fuel valve, fuel filter, fuel flow meter, electric fuel pump. The distance between the filter and meter is 2” and the distance between the meter and elec fuel pump is about 5-6” with a 45 deg offset. When I turn ON the fuel pump the gph increases about 6 gal or so. I believe the meter should be between the engine fuel pump and the carburetor, correct?
Is the extra gph when the electric fuel pump is On going out the fuel overflow line from the engine fuel pump? The electric fuel pump doesn’t have a return line like you would have on the electric fuel injection fuel pumps. It’s an automotive Facet fuel pump.
Thanks

If you are talking about the FT-60 Red Cube the mfg installation says the following:

"If the aircraft has a fuel pump(s), the flow transducer MUST be installed downstream of the last fuel pump.
Installing the transducer upstream of the fuel pump(s) can cause vapor lock and jumpy/inaccurate readings."

Mine is installed as instructed and only changes a decimal point or two when the boost pump is turned on. FWIW.
 
I have the floscan, mounted in the same position as yours. I see about a 1GPH increase in reading when the electric pump is on. The pump is generally only on for brief periods, and I find the total is generally accurate to within 1/2 gallon on each fill up, so I don't worry too much about it. 6GPM seems like a big increase but I'd still be surprised if it's anything but a sensor error.

Chris
 
Floscan

The builder installed a floscan. I just bought the plane and have fueled up 3-4 times and the error difference has been close to 2 gallons, meaning the difference between fuel used and added one time was 1.5 gallons and the next time 3.5 gallons. If it was consistent then I would only need to adjust the K factor.

Does anyone know if the increase in fuel flow when the fuel pump is On is the increase being dumped over board by the engine fuel pump??? As far as a sensor error, shouldn’t the error be consistent whether or not the fuel pump is On or not?? If so and the electric fuel pump is pumping 30 gph then that’s a lot of fuel being wasted during takeoff and while in the pattern.

So should the floscan being installed after the engine fuel pump??
 
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Fuel is not being dumped overboard. The fuel pump vent is only used if the pump diaphragm developes a leak.

You are seeing a higher flow reading because of fuel pulsing in the Floscan when the Facet pump is running. This is a common “feature” even though the flow indication usually doesn’t increase by six gallons. The large change could be due to how your particular system is plumbed.
 
I have the same set-up. The way the pump works creates pulses in the fuel flow. The fuel surges forward, and then slightly retracts when the pressure decreases. This gives an artificially high fuel flow reading because it does not subtract out the reverse flow.
With the pump off I get good readings, but sometimes they jump a bit because the meter is not really reading fuel flow through the carburetor. It is reading fuel flow to the carburetor as the needle valve opens and closes in the bowl. The meter tries to normalize, or average this flow, but it is not perfect. I run my fuel pump only for start-up. I do not use it for take-off or climb, and find my totalizer to be very accurate.
A loss of the primary fuel pump will not result in a sudden engine failure. First you will see a loss of fuel pressure, and then you will have some time before the engine consumes the available fuel in the carburetor bowl. I keep my fuel pressure in my scan on take-off and climb. I have considered an audible alarm for low fuel flow, but have not done anything with that yet.
The other option is to move the fuel flow meter close to the carburetor. The closer it is to the dead-headed end of the run the less pulsing it will see.
 
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Your large difference may be due to the filter just upstream of the floscan, maybe this inhibits the damping of the pulses from the fuel pump, increasing the error. The totalizer error will be larger depending on what percentage of time the boost pump is on. Every 10 minutes it is on in your case will mean an extra 1 gallon “missing”. So if you make lots of short flights where the pump is on a lot, you will see a bigger difference at fill up than if you had made one long flight with minimal boost pump use.

Chris
 
I see the same indication in my non-injected -10. Pump on, flow goes up a fair amount. Following but do not want my red cube forward of the firewall due to heat.
 
People have installed small reservoirs after electric pump before FF sensor to buffer the pulses. There are old RVator articles on this. I never did it, and can't say how well it works. We install FF sensor for convenience and to be on cool side of firewall, not for accuracy. It is common to have FF error with electric AUX pump on.
 
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