Fuel system woes

I suspect the engine is “hot soak flooding” caused by fuel in the float bowls boiling and flooding the engine when it shut down hot.  This is not that uncommon. A fuel return line or fuel coolers won’t help solve this, though they would protect against vapor lock while running.

Mark has the classic symptoms of heat soak flooding, with the worst effects showing up about 20 minutes after shutting down hot. He also mentions his only chance of starting is at wide open throttle, also a tell for heat soak flooding. I would guess after an hour the problem solves itself and the motor will start again. Remedies would include a cooler engine compartment or a fuel with a lower vapor pressure. This is one more consequence of winter blend auto fuel.

Rogere, Roger,

Fuel return (T) to main line supply.

I didn't say it want work, just that it's not an optimum design - return to tank is not just marginally better, it's a lot better, especially for aircraft operating in hot climates.

"If it was thousands of aircraft would be in trouble, but they're good to go."

Lets not forget, a Rotax 912, will run quite happily without a fuel return system.

The fuel return system is to address a phenomena, primarily found in hot climates, fuel boil/vaporisation. This may be exacerbated by ULP petrol supply systems that have winter & summer formulations. The winter formulation, carried over into hot weather months, being more prone to vapourisation.

Your point above means little, if the bulk of the aircraft you referring to are operating in cold/temperate climates, given my statement that you don't even need a fuel return for the 912 to run.

If you want a fuel return, to main supply line, to work at near the return to tank system, consider using some form of fuel cooling, as I mentioned earlier.

😈

Jeff B

"I suspect the engine is “hot soak flooding” caused by fuel in the float bowls boiling and flooding the engine when it shut down hot.  This is not that uncommon.

Agreed. However the management of this phenomena, seem to be up  for debate

"A fuel return line or fuel coolers won’t help solve this, though they would protect against vapor lock while running."

A fuel return line does not solve the heating issue in itself.

The fuel return line, when fuel is pumped (preferably by auxiliary/boost pump), allows for the bulk of hot/vaporised fuel to be sent back to the tank and be replaced by cool liquid fuel. The cool fuel will also help to cool the fuel system & any remaining hot fuel. The liquid fuel will enter the float chamber and the carbys can now perform as designed.

If fuel vaporisation is suspected - Run boost pump, until a steady fuel pressure has been established (if no gauge run for about 3-5 minutes to be sure). Steady fuel pressure, indicates that liquid fuel (few /nil bubbles,rather than vapour) is now being pumped/filled the system. Run boost pump while cranking/starting the engine. Keep the boost pump on, during taxi, (extended) run up checks and TO/Climb Out. Turn off boost, only when safe altitude reached.

"Mark has the classic symptoms of heat soak flooding, with the worst effects showing up about 20 minutes after shutting down hot. He also mentions his only chance of starting is at wide open throttle, also a tell for heat soak flooding. I would guess after an hour the problem solves itself and the motor will start again."

Agreed

"Remedies would include a cooler engine compartment or a fuel with a lower vapor pressure. This is one more consequence of winter blend auto fuel."

On a hot day (say over 40C) a "cooler engine compartment" will only come from a prolonged period on the ground. A viable but tedious, often inconvenient, solution.

Using "a fuel with a lower vapor pressure"  Most reliable would be AvGas, an expensive option, that Rotax 9's tend not to be happy with long term.

I am lucky where I live, I am not subject to the challenge of " winter blend auto fuel.".  I accept that inadvertently using winter blend, on a hot day, is an ongoing problem for N American pilots.

The main problem here is not understanding the merits of (and the best/effcet fitting) of a fuel return system.

A fuel return system is not a solution/fix (engine heating fuel lines) it's a management tool that for the most part actually works.

At the end of the day - for aircraft operating in hot weather FIT A FUEL RETURN SYSTEM.   Any of the, aforementioned, design will be better than non.😈

Try summer fuel, if you can get some these days. Perhaps there is a can stored away somewhere.

Thanks for all the responses!

I'm not sure though if a return line would really help. The heat soaked carb filled with residue fuel would not get replaced by cooler fuel and it would not evaporate through the fuel line, right?

I was thinking of opening the throttle fully after shutting down the engine, perhaps that will give the vapor from the carb an escape path?

Also, I could try closing the main fuel valve before shutting down the engine and wait until the fuel has been depleted from the float chambers. I'll give these options a go.