ORIGINAL: pitstop000
The airframes are Atlanta 60’s, 80’s patternships that were designed around the YS 60’s. The tank location is the problem. The tank is located above the spray bar and causes flooding without a regulator. This is the only location for the tank, even if relocated at the CG it still will be too high. The motor is inverted to make things more challenging.
Best thing to do is sorta what we use for the racing planes. Keep a hemostat/clamp on the fuel line until you start the engine. Once it is running, typically the fuel flow regulates itself.
The idea of mounting the RNVA up top is probably a good approach too. The engine will not mind the position all that much.
More directly to your initial question......
the 90LX does not work well with the regulators. I have used them, and I have gotten them to work. But it takes about an hour of bench time to become familiar with the setup, to get the needles dialed in JUST right, and to ensure the system functions properly. Plug choice becomes critical. I have also used the iron bay regulators... about the same result.
The problem is, sorta what MJD noted. The Jett carb does not "suck" fuel all that much. It is a pressure fed system. Muffler pressure or pipe pressure is needed for fuel delivery to the carb. Those of you who are Jett owners out there know what happens if the muffler pressure line falls off. So, the Cline and Iron Bay regulators struggled with sensing demand. It only worked by mounting the regulator at (pretty much right on) the carb - between the needle and the carb inlet. Now picture what this looks like, the plumbing, physical mounting..... tis not pretty.
I have also had "some" success with the perry pump, used in a 3-line regulated return setup. Again, a bunch of bench time, adjust the engine first, then the pump and return pressure, and then the engine again. My SJ-50 and SJ-90 engines worked ok with this. The LX engines seemed to not like this all that much - pump delivery sometimes led or lagged behind the demand from the engine/pipe.