Fuel system woes

There are Mfg's that run a return line into the gascolator and into fuel supply hose  way back before it gets to the engine compartment into cooler fuel. It doesn't have to go directly into the fuel tank.

Gents,

I was a busy man today. I installed both an electric fuel pump (the Facet one) as well as a drain valve after the mechanical pump. I also insulated the fuel lines (both before and after the mechanical pump).

Well I'm baffled:

I ran the engine until it was nice and warm (CHTs reached 100 C) then shut it down and opened the drain valve. I waited 10 minutes, closed the drain valve and turned on my Efis. Fuel pressure was 0, as expected. I then opened the main fuel selector, used the electrical pump until the pressure was 0.3 bar and tried to start - but she just wouldn't start. Same symptoms as before. Only chance I have is to fully open the throttle.

I have absolutely no issues when she is cold.

Battery is new, plugs are new, carbs were recently serviced.

At this point I don't think I'm looking at vapor lock / flooded cylinders, right? Any thoughts what else this could be?

Thanks

Mark

Mark,

Your story reminds me of the problems all of us classic car enthusiasts have with modern fuel and carbureted cars. You drive the car, then shut it off and it restarts fine in the first 10 minutes. But if you wait 20 minutes it won't start. The remedy is cranking while holding your foot to the floor to keep the throttle plates open and clear out the engine. What is happening (in these cars anyway) is when you shut off the engine and it heats up and transmits that heat to the carburetor, which boils the fuel. This forces fuel vapor into the intake manifold and floods the engine. The carbs are vented, but the fuel vapors still overcome this. The problem goes away completely (in a classic car) with the use of lower vapor pressure fuel like Avgas.

If you a seeing a pressure increase in the fuel line between the pump and carburetor, I don't think that is the problem directly.  And you have proved it's not by installing the valve. But it does indicate to me that your fuel has a high vapor pressure. The 912 carbs are much different than the carb on a classic car, but it makes me wonder if the same thing might be happening. If you are using an automotive fuel, the vapor pressure can be anywhere from 8 to 14 PSI, depending on the requirements in your area for a given season. It would be interesting to try a tank of Avgas, which has a reliable vapor pressure of about 6, and see if the problem goes away. 

I guess that might exactly be the case.

The cowling is really tight on my plane, so basically there is hardly any air passing through with the engine shut down.

Avgas is not really an option as it is ridiculously expensive where I live plus not easily to source unless flying to bigger airports.

I wonder what other options there might be?

"I wonder what other options there might be?"

You may be comming round to fitting a fuel return line, no matter how challenging this may be 😈