Thank you for all your responses!
We did some more testing yesterday, trying to approach it as systematically as possible.
The temperature was again around 25 deg C / 77 deg F.
- Cowling on, boost pump off, we started at around 4.5 psi. As the engine warmed up / time passed, the pressure however dropped to about 3.3 psi and began to fluctuate significantly. When I applied full power over the course of maybe a second or two, the pressure dropped to below 2 psi, fluctuating wildly.
- Cowling off, boost pump off, everything stayed nice and cool, the exact same thing however happened again. Vapor lock therefore indeed doesn't seem to be the issue.
- Turning the boost pump on increased the pressure levels by around 0.3 psi, the overall behavior however stayed the same
- We then replaced the mechanical fuel pump, no change
- Next, we capped the return line. First test, cowling off, the fuel pressure was stable at around 6 psi and stayed there, even after the engine warmed up. Under full power it dropped by about 1 psi, no fluctuations.
- Cowling back on, the pressure strangely stabilized at 5.3 psi and I was getting a brief overpressure alert at around 8.5 psi, when I turned on the boost pump. Some more testing showed again a drop of around 1 psi at full power, no more fluctuations.
- I then took the plane for a test flight, no problems, overall stable fuel pressure other than another brief overpressure alert.
- I am not quite sure about the fwf fuel lines, as they came prefabricated, as a kit. I however used -6AN everywhere else and was getting around 140 l / 37 gallon / hour free flowing at the hose coming out of the gascolator, what should be more than plenty.
- We also tested the fuel flow directly at the carbs, with only the boost pump on. Both were pretty much identical at 74 l / 19 gallons / hour, what should again be way more than enough.
Looking at all of this, I am convinced that somehow the setup with the return line going into the gascolator is the problem.
I am also wondering if the restriction jet, sitting right on top of the gascolator, might cause issues as I understand that most installations have it on the other end, at the fuel manifold.
Today we will look into whether we can find a way to run the return line back into on of the tanks, without ripping the wing apart.
If we can't make this work, a small header tank under the panel, might be a solution: https://www.cxracing.com/FST03-57AN
Thanks again, I will keep you posted!
Oliver
