I have a 914 in a kitfox that I bought last year. When I bought the airplane the fuel pumps were connected in parallel with no check valves. I was seeing wide variations in differential fuel pressure readings when I switched from 1 to 2 pumps on. The most troubling things was that at mid-low power the motor would begin running rough within seconds of having both fuel pumps on. I had no issues running one fuel pump at any power setting and running both pumps seemed to be fine at high power settings.
I have since re-plumbed the fuel pumps so they are in series with check valves in accordance with Rotax's latest instructions. I also replaced the fuel pressure regulator at the advice of a certified rotax repair shop. I initially thought this had solved my rough running problem at low- mid range power. However I've noticed that now it takes a couple of minutes (rather than 20-30 seconds) before the engine begins to run rough. The fastest resolution seems to be when I shut off both fuel pumps for a second or two. The engine almost instantly roars back to smooth running power and then I switch on a single fuel pump and it continues running normally. Again the issue never arises with just one pump on (at any power setting) or at high power settings it runs fine with both pumps on.
It seems to me that I am flooding the carbs, but I thought the pressure regulator should address that.
I should also mention that I rebuilt the carbs about 20 hours ago due to a rough mid range. That seemed to resolve that issue quite well.
