This has been an ongoing issue for years, just do a search on the forum. It has been happening to me for 850 hours, 912uls. It never causes a problem and the engine never hiccups at full power climbouts, but pressure will drop to about 2 psi. I have checked with a mechanical gauge and it agrees with my readout. I do not have an aux pump on my high wing Kitfox; it used to concern me a lot but I have quit worrying about it; it never last too long and then returns to normal. I have the latest fuel pump, return line, and gravity fuel flow tests on the ground with plane in climb attitude show adequate flow, so no blockages. I personally think it is an intermittent flaw in the fuel pump which Rotax refuses to admit; that may be why they recently advised an electric aux pump in all installations. My high wing fuel tanks are probably why it has never caused me a problem. It happens only occasionally and may work perfectly for many flights between incidents.
