FUEL PRESSURE
in 912 / 914 Technical Questions
12 years ago
I finally got around to installing my fuel pressure gauge on my KitFox Vixen 912UL. The gauge is a Holley electric round gauge c/w sender unit mounted just downstream from the original mechanical fuel pump (mid 90`s engine)
In flight the gauge reads a steady 5 psi at all times. Yesterday I shut off the fuel while taxiing in and ran the engine until the pressure bled off to less that 1 psi. I then restarted and instantly got 5 psi again. I then let the plane sit for 15 minutes or so and the pressure remained at 5 psi. After 30 or more minutes the pressure gradually increases to around 10 psi.
I believe what is happening is that the fuel is expanding from heat dissipating from the engine. The fuel cannot escape through the carb float valve or back through the pump so it just pressures up.
Over the years occasionally I have hot start problems about 30 or so minutes after shut down and I always suspected "vapour lock". I now believe it is over-pressured hot fuel blasting into the carbs and FLOODING!!.
I now understand why Rotax is recommending a return fuel line back to the gascolator or tank.
Roger, could you advise what size of restrictor orifice I should have in my return line that I will be installing this week. What about an adjustable valve set to maintain the 5 psi at cruise RPM?
Thanks
Don
In flight the gauge reads a steady 5 psi at all times. Yesterday I shut off the fuel while taxiing in and ran the engine until the pressure bled off to less that 1 psi. I then restarted and instantly got 5 psi again. I then let the plane sit for 15 minutes or so and the pressure remained at 5 psi. After 30 or more minutes the pressure gradually increases to around 10 psi.
I believe what is happening is that the fuel is expanding from heat dissipating from the engine. The fuel cannot escape through the carb float valve or back through the pump so it just pressures up.
Over the years occasionally I have hot start problems about 30 or so minutes after shut down and I always suspected "vapour lock". I now believe it is over-pressured hot fuel blasting into the carbs and FLOODING!!.
I now understand why Rotax is recommending a return fuel line back to the gascolator or tank.
Roger, could you advise what size of restrictor orifice I should have in my return line that I will be installing this week. What about an adjustable valve set to maintain the 5 psi at cruise RPM?
Thanks
Don
