912 ULS: Partial power loss on takeoff - vapor lock?

After you start the engine for a few minutes and advance the throttle quickly does this same thing happen? Does it make a difference if the electric pump is on or off?

I don't think its vapor lock after using fire sleeve and header wrap. Plus fuel coming into the gascolator is cool from the tanks. We're not talking about boil fuel temps here. Plus that 2 psi on a high wing isn't usually normal.

Things to rule in or out,

I'd be looking for something else that's hindering the flow. Are your fuel hoses up to the mechanical pump at least 5/16"? It shouldn't be only 1/4". Even if the hoses are okay size wise, but you have smaller fittings in the line that slows flow. Maybe a fuel hose that has too much of a bend or try not advancing the throttle too quickly. Your electric auxiliary pump may be causing a momentary slow flow if it's not on?

I've seen numerous aircraft where the return fuel line goes into the gascolator. Like the Flight Design CT's. They don't have an issue and I've seen others with no issues too. Plus like you said other aircraft like yours don't have an issue.

p.s.

Do not sit and idle down at that 1600 rpm area. If you're going to just sit and idle keep it at 2K+.

You can identify vapor lock by monitoring your fuel flow. If you start seeing spikes to 9 GPH or higher, that is a clear indication that vapor lock is occurring.

That said, it does appear that you have restricted fuel line fittings, especially the one coming out of the electric fuel pump. Also, ensure that your fuel tank venting is adequate.

Also, make sure the fuel pump drain line isn't positioned in the slipstream, as this can negatively affect the fuel pump and cause low fuel pressure.

You are using Winter car gas (high vapor pressure) on a warm day; this only happens when heat soaked; fuel return line goes to gascolator. As others have suggested, I think this is a fuel vapor problem.  Drain the fuel and put in Avgas, if the problem goes away you have your answer.  Many of us use a 50/50 mix of avgas and mogas when only winter fuel is available here in the US.  If I’m wrong, the only cost is some avgas. 

Hi Oliver,

I have asked a friend on your behalf about this, I have recently heard about vapor lock and something to do with it sitting in the cross over supply tube. This pilot after stopping for a while can’t restart unless he turns off all fuel pumps, he has a 914, and this way he can start it, then introduce fuel pumps. With your mechanical fuel pump I see no option but a return to a tank which mixes with cool fresh fuel, like a Skytek header tank, a Canadian company. Their header tank is easy to attach to the B pillar behind the door, holds about 5 liters I recall, and the return line goes into the top as you’d expect. I tried to see if I could PM you here and over at the Zenith Builders Forum but couldn’t find you, could you contact me at bfreeppg@gmail.com so we can discuss further, I’m building a 914 powered Cruzer too. Hope this helps., a little.

Roger my friend,

I am gobsmacked!

"I've seen numerous aircraft where the return fuel line goes into the gascolator. Like the Flight Design CT's. They don't have an issue and I've seen others with no issues too. Plus like you said other aircraft like yours don't have an issue."

Just because some sloppy manufacturer/home builders do this, does not make it right - it is clearly wrong and must compromise the concept/effectiveness of a fuel return system.😈