Fuel flow varying/wondering during cruise

[FireMedic_2009]

When I initially bought the plane in April of last year the flo scan was mounted right after the booster pump on the floorboard and produced big variations in fuel flow when the boost pump was On. So I’m mounted it on the firewall attaching it to the engine mount with adel clamps and is pretty accurate on total fuel used. It is mounted after the mechanical fuel pump and used braided hose to connect it between the mechanical fuel pump and the carburetor. Coming off the mechanical fuel pump I swept the hose towards the right side of the firewall and attached the flow scan about 4 inches after the bend coming back towards the left and then made a sweeping bend coming off the flo scan to the carburetor . The flo scan’s height might be a slightly above the carburetor to keep some space between the exhaust pipes. The fuel flow meanders whether the boost pump is on or off by 0.5gph or so. Didn’t know if flo scans are finicky about not having 6 inches of straight hose before and after the unit.

I just didn’t know if this is typical on carbureted engines since my fuel injected engine on my 6A, which used a red cube, was pretty rock solid.

Thanks again!

[bonefishdave]

I was having similar fluctuations and noticed it even with the engine off and mixture idle cutoff.

Turns out I had my transponder / ADS-B antenna mounted forward near the firewall. As soon as I turned off the transponder it stopped. Ended up moving the antenna back behind the aft bulkhead and that solved it.

[airguy]

Originally Posted by bonefishdave

I was having similar fluctuations and noticed it even with the engine off and mixture idle cutoff.

Turns out I had my transponder / ADS-B antenna mounted forward near the firewall. As soon as I turned off the transponder it stopped. Ended up moving the antenna back behind the aft bulkhead and that solved it.

RFI will drive you to drink.

[PilotjohnS]

Originally Posted by airguy

RFI will drive you to drink.

So that is my problem....

[Bavafa]

Originally Posted by Carl Froehlich

How the fuel flow sender is mounted makes a difference. The rules I stick to:
- Do not mount on the engine. This goes against how Van’s tells you to mount it on the RV-14 - which I had first hand experience with huge fuel flow measurement issues. After moving the sender off the engine the problem was resolved.
- Mount it down stream of the mechanical fuel pump. On the first RV-10 I mounted it in the tunnel like Van’s says to do (between the boost and mechanical fuel pumps). It was accurate unless the boost pump was on - then it shot up 1-2 GPH.
- Mount it so the hose run from the sender to either the carb or fuel manager is up hill. Need not be much, but up hill.
- No 90 degree fittings - other than the nice smooth tube kind.

Of late I mount the sender on the engine mount using Adel clamps.

Carl

As many have reported, there are a lot of varying factor and one's experience differs from another.
My last RV14 had the Red cube mounted per VANS instruction and it was rock solid. My new 14 had it mounted to the case and it was pretty solid but I had to change the k-factor for more accuracy. I have since changed the mount to the case with some rubber in between as a means to reduce any vibration and the result has not been as steady as before but I need more hours to say for sure.

[rvsxer]

My setup is identical to Paul Dye's (mine has a carb) and my indications are identical, right down to the higher flow with the boost pump on. My totalizer is always within .1-.2 gallon at fillup. I installed the Red Cube in 2012 along with a legacy G3X system. Back then the instructions didn't mention installation downstream of the engine driven pump, either. I'm satisfied with what I have.

[rvsxer]

Originally Posted by lr172

Both of my planes have the cube downstream and flow is solid as a rock in cruise.

Larry

Of course they are, you are running fuel injection...

[lr172]

Originally Posted by rvsxer

Of course they are, you are running fuel injection...

It was also downstream of the pumps when it was a carb'ed as well and was also rock solid. The recommendation is that the cubes are downstream from the pumps. No recommendation that it is downstream of the FI servo, though I and others often do this also.

Larry