912 UL in an early Kitfox and fuel return lines

Bill,

The check valve is needed because of the header tank installed in the Kitfox.  Must keep fuel from flowing backward when the engine is stopped.  The good folks at Kitfox said so.  

Ken - I got confused by all the different airplanes I work on.  This aircraft is a Kitfox Vixen Model 5 and has a 912UL, not a 914.  Another Kitfox on the field has a 914.  Sorry about the confusion. 

I did take your advice and looked up the fuel system requirements in the 912 Installation Manual and found a diagram showing a restrictor jet in the fuel return line Item #9 on the diagram and again Item #6 in the Fuel Manifold graphic.  This orifice is listed as 0.35mm (0.014in).  This engine does not use the Rotax fuel manifold but had a simple T-fitting in the fuel lines between the fuel pump and carbs.  At idle the new style fuel pump installed on this engine would overfill the float bowls and cause rough running not to mention fuel pouring out of the float bowl vent tubes.  I changed out the T for a 4-way and engineered a return line with a 35 Pilot Jet #963-820 to maintain fuel pressure in the lines instead of the fuel just flowing back to the header tank.  The engine starts easily, idles smoothly and on the ground runs up to full power.  Now comes the fun part.  Upon takeoff on the climb out the engine would begin to run roughly as if it was fuel starved and we would make the impossible turn and return opposite direction to land.  Checks for water or debris in the fuel turned up nothing.  We tied the aircraft down and did several full power runs and on only one occasion the engine began to run rough just like it had on the climb out, but only once.  This aircraft has no fuel pressure gauge so I don't know what is going on there.  We are either brave or stupid, between these tests and checks we tried twice more to takeoff, and each time just about as we thought the problem had been a fluke and corrected itself the engine would lose power and run rough again.  At least the owner/test pilot is a good stick and rudder man and I'm a brave foolhardy mechanic because we got to practice three impossible turn emergency procedures.  While I am testing the fuel pressure does anyone have any ideas why she runs great on the ground and looses power on climb out?   Pictures and Rotax IM pages attached.  

I would guess that the difference is that the plane sits more level on the ground and more Nose-Up in the climb.

Do you have the time to notice if the engine power returns when diving back to the runway or reducing to cruise power clears up the misfires???

Is there an AUX fuel pump in the mix?

Yes, I did notice the power loss after take-off when the nose was pitched up to climb and yes the engine seemed to smooth out when the throttle was reduced and the nose pitched down to flatten out the turn back for the runway.  And there is an aux fuel pump and the first take-off was without the pump on, but subsequent take-offs were with the pump on.  

I guess fuel filters are next on the hit list.

Alternately,  pull the hoses off the carbs and confirm that the AUX Pump can push ~1/2 Gal per minute into a clean container.