fuel flow from left and right tank

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2 years 6 months ago #1840 by STEVE ELLS
Replied by STEVE ELLS on topic fuel flow from left and right tank
You're experiencing a very common problem. The vent for the left and right fuel tanks is the L shaped tube behind the outer end of the left wing strut. And a crossover tube from the upper inner end of the left fuel tank across the fuselage to the upper inboard end of the right fuel tank. In theory the vent should provide venting for the left and right tank simultaneously.
But because of wing dihedral--where the wing tip is farther of the ground than the inner end of the fuel tanks, the vent interconnect tube is blocked by fuel when the tanks are full. So the pressure from the vent tube pushes the fuel in the left tank long before the vent tube is no longer blocked by the fuel at the inner end of the fuel tank.
Only after the vent tube interconnect is uncovered by fuel does the venting clear to the right tank. By then the fuel for the engine has been solely from the left tank.
Cessna issued single engine bulletin SEB 81-08 titled fuel system venting adjustment; it was superseded by SEB 92

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-27 (which I have attached). This bulletin provides the option to installed vented fuel caps on both fuel fillers, which may help alleviate your flow problems. The caps called out in this bulletin for your 175 are part number C156003-0101. These caps are available from a variety of vendors (Aircraft Spruce; MacFarlane, eBay, etc) at a new cost of around $150.
The other hint is to make sure the tank vent tube behind the wing strut is adjusted for position in accordance with the service manual.
Please let me know what you find works best.
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2 years 6 months ago #1830 by Charles Gill
fuel flow from left and right tank was created by Charles Gill
When in flight the fuel flow from the left tank is three times as much as from the right tank when the selector switch is on both. When on the ground and draining the tanks with the fuel line from the fuel line to the engine compartment the flow rate is the same form both tanks. I have timed how long it takes to drain 20 gallons from each tank. There is good cross venting between the tanks. I have not allowed the left tank to run dry in flight while on selector is on both as I am not sure what will happen. The aircraft is a 1962 cessna 175C with a Franklin 335 engine and a constant speed prop. Plane preforms great but I cannot figure out the different flow rates from the tanks in flight. Any suggestions?

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