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Old 01-17-2018, 06:13 PM
YellowJacket RV9 YellowJacket RV9 is offline
Join Date: Oct 2012
Location: Clearwater, FL / KZPH
Posts: 1,141

Originally Posted by gear1 View Post
If the red cube is upstream of the pump it would be normal to have fluctuating fuel flow. The engine driven fuel pump has an intermittent draw, in that the cam operated lever is pushed against the diaphragm spring to recharge the pumping chamber, and then the spring slowly pushes the fuel out of the pumping chamber towards the carburetor. During the fuel pump discharge phase no fuel passes through the red cube. To create a stable fuel flow signal, place the red cube between the fuel pump and carburetor.
Just had the same thought. I wonder why it seems to occur more to those of us running carbs - the fuel pumps are essentially the same besides the spring I thought.

Chris Johnson
RV-9A - Done(ish) 4/5/16! Flying 4/7/16
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Old 01-17-2018, 06:38 PM
Finley Atherton Finley Atherton is offline
Join Date: Nov 2005
Posts: 703

Another one with this issue on my 0-320. Over the years I have tried just about everything. The best solution was installing the FC-10 Fuel Computer as it has a selectable electronic dampening function which most fuel computers do not have. This makes the flow reading more stable (not perfect) but slower to react.

For my aircraft the actual cause of the fluctatuating reading are changing G forces caused by turbulence. In smooth air the readings are always stable.

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Old 01-18-2018, 01:20 AM
dougmattson dougmattson is offline
Join Date: Sep 2007
Location: Dike Iowa
Posts: 98
Default red cube

On my O-320 the red cube is between the pumps and the carb. If the flow drops below about 6 gpm the numbers will walk up and down about a gal. If I run it so the flow is above 7 it gets a lot steadier. I think it is the float bowl filling also.
9A flying
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